| Message ID | 20210623150143.188184-3-mreitz@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block: block-status cache for data regions | expand |
23.06.2021 18:01, Max Reitz wrote: > As we have attempted before > (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, > "file-posix: Cache lseek result for data regions"; > https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, > "file-posix: Cache next hole"), this patch seeks to reduce the number of > SEEK_DATA/HOLE operations the file-posix driver has to perform. The > main difference is that this time it is implemented as part of the > general block layer code. > > The problem we face is that on some filesystems or in some > circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the > implementation is outside of qemu, there is little we can do about its > performance. > > We have already introduced the want_zero parameter to > bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls > unless we really want zero information; but sometimes we do want that > information, because for files that consist largely of zero areas, > special-casing those areas can give large performance boosts. So the > real problem is with files that consist largely of data, so that > inquiring the block status does not gain us much performance, but where > such an inquiry itself takes a lot of time. > > To address this, we want to cache data regions. Most of the time, when > bad performance is reported, it is in places where the image is iterated > over from start to end (qemu-img convert or the mirror job), so a simple > yet effective solution is to cache only the current data region. > > (Note that only caching data regions but not zero regions means that > returning false information from the cache is not catastrophic: Treating > zeroes as data is fine. While we try to invalidate the cache on zero > writes and discards, such incongruences may still occur when there are > other processes writing to the image.) > > We only use the cache for nodes without children (i.e. protocol nodes), > because that is where the problem is: Drivers that rely on block-status > implementations outside of qemu (e.g. SEEK_DATA/HOLE). > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 > Signed-off-by: Max Reitz <mreitz@redhat.com> I'm new to RCU, so my review can't be reliable.. > --- > include/block/block_int.h | 47 ++++++++++++++++++++++ > block.c | 84 +++++++++++++++++++++++++++++++++++++++ > block/io.c | 61 ++++++++++++++++++++++++++-- > 3 files changed, 189 insertions(+), 3 deletions(-) > > diff --git a/include/block/block_int.h b/include/block/block_int.h > index a8f9598102..fcb599dd1c 100644 > --- a/include/block/block_int.h > +++ b/include/block/block_int.h > @@ -832,6 +832,22 @@ struct BdrvChild { > QLIST_ENTRY(BdrvChild) next_parent; > }; > > +/* > + * Allows bdrv_co_block_status() to cache one data region for a > + * protocol node. > + * > + * @valid: Whether the cache is valid (should be accessed with atomic > + * functions so this can be reset by RCU readers) > + * @data_start: Offset where we know (or strongly assume) is data > + * @data_end: Offset where the data region ends (which is not necessarily > + * the start of a zeroed region) > + */ > +typedef struct BdrvBlockStatusCache { > + bool valid; > + int64_t data_start; > + int64_t data_end; > +} BdrvBlockStatusCache; > + > struct BlockDriverState { > /* Protected by big QEMU lock or read-only after opening. No special > * locking needed during I/O... > @@ -997,6 +1013,11 @@ struct BlockDriverState { > > /* BdrvChild links to this node may never be frozen */ > bool never_freeze; > + > + /* Lock for block-status cache RCU writers */ > + CoMutex bsc_modify_lock; > + /* Always non-NULL, but must only be dereferenced under an RCU read guard */ > + BdrvBlockStatusCache *block_status_cache;> }; > > struct BlockBackendRootState { > @@ -1422,4 +1443,30 @@ static inline BlockDriverState *bdrv_primary_bs(BlockDriverState *bs) > */ > void bdrv_drain_all_end_quiesce(BlockDriverState *bs); > > +/** > + * Check whether the given offset is in the cached block-status data > + * region. > + * > + * If it is, and @pnum is not NULL, *pnum is set to > + * `bsc.data_end - offset`, i.e. how many bytes, starting from > + * @offset, are data (according to the cache). > + * Otherwise, *pnum is not touched. > + */ > +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum); > + > +/** > + * If [offset, offset + bytes) overlaps with the currently cached > + * block-status region, invalidate the cache. > + * > + * (To be used by I/O paths that cause data regions to be zero or > + * holes.) > + */ > +void bdrv_bsc_invalidate_range(BlockDriverState *bs, > + int64_t offset, int64_t bytes); > + > +/** > + * Mark the range [offset, offset + bytes) as a data region. > + */ > +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes); > + > #endif /* BLOCK_INT_H */ > diff --git a/block.c b/block.c > index 3f456892d0..9ab9459f7a 100644 > --- a/block.c > +++ b/block.c > @@ -49,6 +49,8 @@ > #include "qemu/timer.h" > #include "qemu/cutils.h" > #include "qemu/id.h" > +#include "qemu/range.h" > +#include "qemu/rcu.h" > #include "block/coroutines.h" > > #ifdef CONFIG_BSD > @@ -398,6 +400,9 @@ BlockDriverState *bdrv_new(void) > > qemu_co_queue_init(&bs->flush_queue); > > + qemu_co_mutex_init(&bs->bsc_modify_lock); > + bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1); > + > for (i = 0; i < bdrv_drain_all_count; i++) { > bdrv_drained_begin(bs); > } > @@ -4635,6 +4640,8 @@ static void bdrv_close(BlockDriverState *bs) > bs->explicit_options = NULL; > qobject_unref(bs->full_open_options); > bs->full_open_options = NULL; > + g_free(bs->block_status_cache); > + bs->block_status_cache = NULL; > > bdrv_release_named_dirty_bitmaps(bs); > assert(QLIST_EMPTY(&bs->dirty_bitmaps)); > @@ -7590,3 +7597,80 @@ BlockDriverState *bdrv_backing_chain_next(BlockDriverState *bs) > { > return bdrv_skip_filters(bdrv_cow_bs(bdrv_skip_filters(bs))); > } > + > +/** > + * Check whether [offset, offset + bytes) overlaps with the cached > + * block-status data region. > + * > + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`, > + * which is what bdrv_bsc_is_data()'s interface needs. > + * Otherwise, *pnum is not touched. > + */ > +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs, > + int64_t offset, int64_t bytes, > + int64_t *pnum) > +{ > + BdrvBlockStatusCache *bsc = bs->block_status_cache; Shouldn't use qatomic_rcu_read() ? > + bool overlaps; > + > + overlaps = > + qatomic_read(&bsc->valid) && Hmm. Why you need atomic access? I thought that after getting rcu pointer, we are safe to read the fields. Ah, I see, you want to also set it in rcu-reader code.. Isn't it better just to do normal rcu-write and set pointer to NULL, when cache becomes invalid? I think keeping heap-allocated structure with valid=false inside doesn't make much sense. > + ranges_overlap(offset, bytes, bsc->data_start, > + bsc->data_end - bsc->data_start); > + > + if (overlaps && pnum) { > + *pnum = bsc->data_end - offset; > + } > + > + return overlaps; > +} > + > +/** > + * See block_int.h for this function's documentation. > + */ > +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum) > +{ > + bool overlaps; > + > + WITH_RCU_READ_LOCK_GUARD() { > + overlaps = bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum); > + } > + > + return overlaps; > +} this may be written simpler I think: RCU_READ_LOCK_GUARD(); return bdrv_bsc_range_overlaps_locked(..); > + > +/** > + * See block_int.h for this function's documentation. > + */ > +void bdrv_bsc_invalidate_range(BlockDriverState *bs, > + int64_t offset, int64_t bytes) > +{ > + WITH_RCU_READ_LOCK_GUARD() { > + if (bdrv_bsc_range_overlaps_locked(bs, offset, bytes, NULL)) { > + qatomic_set(&bs->block_status_cache->valid, false); > + } > + } > +} Same here, why not use RCU_READ_LOCK_GUARD() ? > + > +/** > + * See block_int.h for this function's documentation. > + */ > +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes) > +{ > + BdrvBlockStatusCache *new_bsc = g_new(BdrvBlockStatusCache, 1); > + BdrvBlockStatusCache *old_bsc; > + > + *new_bsc = (BdrvBlockStatusCache) { > + .valid = true, > + .data_start = offset, > + .data_end = offset + bytes, > + }; > + > + WITH_QEMU_LOCK_GUARD(&bs->bsc_modify_lock) { > + old_bsc = bs->block_status_cache; > + qatomic_rcu_set(&bs->block_status_cache, new_bsc); > + synchronize_rcu(); Interesting, that until this, synchronize_rcu() is used only in tests.. (I tried to search examples of rcu writing in the code) > + } > + > + g_free(old_bsc); > +} > diff --git a/block/io.c b/block/io.c > index 323854d063..85fa449bf9 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -1878,6 +1878,9 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, > return -ENOTSUP; > } > > + /* Invalidate the cached block-status data range if this write overlaps */ > + bdrv_bsc_invalidate_range(bs, offset, bytes); > + > assert(alignment % bs->bl.request_alignment == 0); > head = offset % alignment; > tail = (offset + bytes) % alignment; > @@ -2442,9 +2445,58 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs, > aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset; > > if (bs->drv->bdrv_co_block_status) { > - ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, > - aligned_bytes, pnum, &local_map, > - &local_file); > + bool from_cache = false; > + > + /* > + * Use the block-status cache only for protocol nodes: Format > + * drivers are generally quick to inquire the status, but protocol > + * drivers often need to get information from outside of qemu, so > + * we do not have control over the actual implementation. There > + * have been cases where inquiring the status took an unreasonably > + * long time, and we can do nothing in qemu to fix it. > + * This is especially problematic for images with large data areas, > + * because finding the few holes in them and giving them special > + * treatment does not gain much performance. Therefore, we try to > + * cache the last-identified data region. > + * > + * Second, limiting ourselves to protocol nodes allows us to assume > + * the block status for data regions to be DATA | OFFSET_VALID, and > + * that the host offset is the same as the guest offset. > + * > + * Note that it is possible that external writers zero parts of > + * the cached regions without the cache being invalidated, and so > + * we may report zeroes as data. This is not catastrophic, > + * however, because reporting zeroes as data is fine. > + */ > + if (QLIST_EMPTY(&bs->children)) { > + if (bdrv_bsc_is_data(bs, aligned_offset, pnum)) { > + ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID; > + local_file = bs; > + local_map = aligned_offset; > + > + from_cache = true; > + } > + } > + > + if (!from_cache) { > + ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, > + aligned_bytes, pnum, &local_map, > + &local_file); > + > + /* > + * Note that checking QLIST_EMPTY(&bs->children) is also done when > + * the cache is queried above. Technically, we do not need to check > + * it here; the worst that can happen is that we fill the cache for > + * non-protocol nodes, and then it is never used. However, filling > + * the cache requires an RCU update, so double check here to avoid > + * such an update if possible. > + */ > + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && > + QLIST_EMPTY(&bs->children)) > + { > + bdrv_bsc_fill(bs, aligned_offset, *pnum); > + } > + } > } else { > /* Default code for filters */ > > @@ -2997,6 +3049,9 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset, > return 0; > } > > + /* Invalidate the cached block-status data range if this discard overlaps */ > + bdrv_bsc_invalidate_range(bs, offset, bytes); > + > /* Discard is advisory, but some devices track and coalesce > * unaligned requests, so we must pass everything down rather than > * round here. Still, most devices will just silently ignore >
On 24.06.21 12:05, Vladimir Sementsov-Ogievskiy wrote: > 23.06.2021 18:01, Max Reitz wrote: >> As we have attempted before >> (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, >> "file-posix: Cache lseek result for data regions"; >> https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, >> "file-posix: Cache next hole"), this patch seeks to reduce the number of >> SEEK_DATA/HOLE operations the file-posix driver has to perform. The >> main difference is that this time it is implemented as part of the >> general block layer code. >> >> The problem we face is that on some filesystems or in some >> circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the >> implementation is outside of qemu, there is little we can do about its >> performance. >> >> We have already introduced the want_zero parameter to >> bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls >> unless we really want zero information; but sometimes we do want that >> information, because for files that consist largely of zero areas, >> special-casing those areas can give large performance boosts. So the >> real problem is with files that consist largely of data, so that >> inquiring the block status does not gain us much performance, but where >> such an inquiry itself takes a lot of time. >> >> To address this, we want to cache data regions. Most of the time, when >> bad performance is reported, it is in places where the image is iterated >> over from start to end (qemu-img convert or the mirror job), so a simple >> yet effective solution is to cache only the current data region. >> >> (Note that only caching data regions but not zero regions means that >> returning false information from the cache is not catastrophic: Treating >> zeroes as data is fine. While we try to invalidate the cache on zero >> writes and discards, such incongruences may still occur when there are >> other processes writing to the image.) >> >> We only use the cache for nodes without children (i.e. protocol nodes), >> because that is where the problem is: Drivers that rely on block-status >> implementations outside of qemu (e.g. SEEK_DATA/HOLE). >> >> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 >> Signed-off-by: Max Reitz <mreitz@redhat.com> > > I'm new to RCU, so my review can't be reliable.. Yeah, well, same here. :) >> --- >> include/block/block_int.h | 47 ++++++++++++++++++++++ >> block.c | 84 +++++++++++++++++++++++++++++++++++++++ >> block/io.c | 61 ++++++++++++++++++++++++++-- >> 3 files changed, 189 insertions(+), 3 deletions(-) >> >> diff --git a/include/block/block_int.h b/include/block/block_int.h >> index a8f9598102..fcb599dd1c 100644 >> --- a/include/block/block_int.h >> +++ b/include/block/block_int.h >> @@ -832,6 +832,22 @@ struct BdrvChild { >> QLIST_ENTRY(BdrvChild) next_parent; >> }; >> +/* >> + * Allows bdrv_co_block_status() to cache one data region for a >> + * protocol node. >> + * >> + * @valid: Whether the cache is valid (should be accessed with atomic >> + * functions so this can be reset by RCU readers) >> + * @data_start: Offset where we know (or strongly assume) is data >> + * @data_end: Offset where the data region ends (which is not >> necessarily >> + * the start of a zeroed region) >> + */ >> +typedef struct BdrvBlockStatusCache { >> + bool valid; >> + int64_t data_start; >> + int64_t data_end; >> +} BdrvBlockStatusCache; >> + >> struct BlockDriverState { >> /* Protected by big QEMU lock or read-only after opening. No >> special >> * locking needed during I/O... >> @@ -997,6 +1013,11 @@ struct BlockDriverState { >> /* BdrvChild links to this node may never be frozen */ >> bool never_freeze; >> + >> + /* Lock for block-status cache RCU writers */ >> + CoMutex bsc_modify_lock; >> + /* Always non-NULL, but must only be dereferenced under an RCU >> read guard */ >> + BdrvBlockStatusCache *block_status_cache;> }; >> struct BlockBackendRootState { >> @@ -1422,4 +1443,30 @@ static inline BlockDriverState >> *bdrv_primary_bs(BlockDriverState *bs) >> */ >> void bdrv_drain_all_end_quiesce(BlockDriverState *bs); >> +/** >> + * Check whether the given offset is in the cached block-status data >> + * region. >> + * >> + * If it is, and @pnum is not NULL, *pnum is set to >> + * `bsc.data_end - offset`, i.e. how many bytes, starting from >> + * @offset, are data (according to the cache). >> + * Otherwise, *pnum is not touched. >> + */ >> +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t >> *pnum); >> + >> +/** >> + * If [offset, offset + bytes) overlaps with the currently cached >> + * block-status region, invalidate the cache. >> + * >> + * (To be used by I/O paths that cause data regions to be zero or >> + * holes.) >> + */ >> +void bdrv_bsc_invalidate_range(BlockDriverState *bs, >> + int64_t offset, int64_t bytes); >> + >> +/** >> + * Mark the range [offset, offset + bytes) as a data region. >> + */ >> +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t >> bytes); >> + >> #endif /* BLOCK_INT_H */ >> diff --git a/block.c b/block.c >> index 3f456892d0..9ab9459f7a 100644 >> --- a/block.c >> +++ b/block.c >> @@ -49,6 +49,8 @@ >> #include "qemu/timer.h" >> #include "qemu/cutils.h" >> #include "qemu/id.h" >> +#include "qemu/range.h" >> +#include "qemu/rcu.h" >> #include "block/coroutines.h" >> #ifdef CONFIG_BSD >> @@ -398,6 +400,9 @@ BlockDriverState *bdrv_new(void) >> qemu_co_queue_init(&bs->flush_queue); >> + qemu_co_mutex_init(&bs->bsc_modify_lock); >> + bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1); >> + >> for (i = 0; i < bdrv_drain_all_count; i++) { >> bdrv_drained_begin(bs); >> } >> @@ -4635,6 +4640,8 @@ static void bdrv_close(BlockDriverState *bs) >> bs->explicit_options = NULL; >> qobject_unref(bs->full_open_options); >> bs->full_open_options = NULL; >> + g_free(bs->block_status_cache); >> + bs->block_status_cache = NULL; >> bdrv_release_named_dirty_bitmaps(bs); >> assert(QLIST_EMPTY(&bs->dirty_bitmaps)); >> @@ -7590,3 +7597,80 @@ BlockDriverState >> *bdrv_backing_chain_next(BlockDriverState *bs) >> { >> return bdrv_skip_filters(bdrv_cow_bs(bdrv_skip_filters(bs))); >> } >> + >> +/** >> + * Check whether [offset, offset + bytes) overlaps with the cached >> + * block-status data region. >> + * >> + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`, >> + * which is what bdrv_bsc_is_data()'s interface needs. >> + * Otherwise, *pnum is not touched. >> + */ >> +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs, >> + int64_t offset, int64_t >> bytes, >> + int64_t *pnum) >> +{ >> + BdrvBlockStatusCache *bsc = bs->block_status_cache; > > Shouldn't use qatomic_rcu_read() ? Oh, right, probably so. > >> + bool overlaps; >> + >> + overlaps = >> + qatomic_read(&bsc->valid) && > > Hmm. Why you need atomic access? I thought that after getting rcu > pointer, we are safe to read the fields. > > Ah, I see, you want to also set it in rcu-reader code.. > > Isn't it better just to do normal rcu-write and set pointer to NULL, > when cache becomes invalid? I think keeping heap-allocated structure > with valid=false inside doesn't make much sense. It does, because this way I don’t need an expensive RCU write. >> + ranges_overlap(offset, bytes, bsc->data_start, >> + bsc->data_end - bsc->data_start); >> + >> + if (overlaps && pnum) { >> + *pnum = bsc->data_end - offset; >> + } >> + >> + return overlaps; >> +} >> + >> +/** >> + * See block_int.h for this function's documentation. >> + */ >> +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t >> *pnum) >> +{ >> + bool overlaps; >> + >> + WITH_RCU_READ_LOCK_GUARD() { >> + overlaps = bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum); >> + } >> + >> + return overlaps; >> +} > > this may be written simpler I think: > > RCU_READ_LOCK_GUARD(); > return bdrv_bsc_range_overlaps_locked(..); Hm, I’ll see whether it grows on me. I kind of like the explicit scope, even if it’s longer. >> + >> +/** >> + * See block_int.h for this function's documentation. >> + */ >> +void bdrv_bsc_invalidate_range(BlockDriverState *bs, >> + int64_t offset, int64_t bytes) >> +{ >> + WITH_RCU_READ_LOCK_GUARD() { >> + if (bdrv_bsc_range_overlaps_locked(bs, offset, bytes, NULL)) { >> + qatomic_set(&bs->block_status_cache->valid, false); >> + } >> + } >> +} > > Same here, why not use RCU_READ_LOCK_GUARD() ? > >> + >> +/** >> + * See block_int.h for this function's documentation. >> + */ >> +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes) >> +{ >> + BdrvBlockStatusCache *new_bsc = g_new(BdrvBlockStatusCache, 1); >> + BdrvBlockStatusCache *old_bsc; >> + >> + *new_bsc = (BdrvBlockStatusCache) { >> + .valid = true, >> + .data_start = offset, >> + .data_end = offset + bytes, >> + }; >> + >> + WITH_QEMU_LOCK_GUARD(&bs->bsc_modify_lock) { >> + old_bsc = bs->block_status_cache; >> + qatomic_rcu_set(&bs->block_status_cache, new_bsc); >> + synchronize_rcu(); > > Interesting, that until this, synchronize_rcu() is used only in > tests.. (I tried to search examples of rcu writing in the code) Well, as far as I understood the docs, synchronize_rcu() is a thing that can be used, besides call_rcu(). I didn’t want to use call_rcu(), because it requires adding an rcu_head struct to the protected object... Now that I look closer at the docs, it says "it is better" to release all locks before synchronize_rcu(), including the BQL. Perhaps I should give call_rcu() a try after all. Max > > >> + } >> + >> + g_free(old_bsc); >> +} >> diff --git a/block/io.c b/block/io.c >> index 323854d063..85fa449bf9 100644 >> --- a/block/io.c >> +++ b/block/io.c >> @@ -1878,6 +1878,9 @@ static int coroutine_fn >> bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, >> return -ENOTSUP; >> } >> + /* Invalidate the cached block-status data range if this write >> overlaps */ >> + bdrv_bsc_invalidate_range(bs, offset, bytes); >> + >> assert(alignment % bs->bl.request_alignment == 0); >> head = offset % alignment; >> tail = (offset + bytes) % alignment; >> @@ -2442,9 +2445,58 @@ static int coroutine_fn >> bdrv_co_block_status(BlockDriverState *bs, >> aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset; >> if (bs->drv->bdrv_co_block_status) { >> - ret = bs->drv->bdrv_co_block_status(bs, want_zero, >> aligned_offset, >> - aligned_bytes, pnum, >> &local_map, >> - &local_file); >> + bool from_cache = false; >> + >> + /* >> + * Use the block-status cache only for protocol nodes: Format >> + * drivers are generally quick to inquire the status, but >> protocol >> + * drivers often need to get information from outside of >> qemu, so >> + * we do not have control over the actual implementation. >> There >> + * have been cases where inquiring the status took an >> unreasonably >> + * long time, and we can do nothing in qemu to fix it. >> + * This is especially problematic for images with large data >> areas, >> + * because finding the few holes in them and giving them >> special >> + * treatment does not gain much performance. Therefore, we >> try to >> + * cache the last-identified data region. >> + * >> + * Second, limiting ourselves to protocol nodes allows us to >> assume >> + * the block status for data regions to be DATA | >> OFFSET_VALID, and >> + * that the host offset is the same as the guest offset. >> + * >> + * Note that it is possible that external writers zero parts of >> + * the cached regions without the cache being invalidated, >> and so >> + * we may report zeroes as data. This is not catastrophic, >> + * however, because reporting zeroes as data is fine. >> + */ >> + if (QLIST_EMPTY(&bs->children)) { >> + if (bdrv_bsc_is_data(bs, aligned_offset, pnum)) { >> + ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID; >> + local_file = bs; >> + local_map = aligned_offset; >> + >> + from_cache = true; >> + } >> + } >> + >> + if (!from_cache) { >> + ret = bs->drv->bdrv_co_block_status(bs, want_zero, >> aligned_offset, >> + aligned_bytes, pnum, >> &local_map, >> + &local_file); >> + >> + /* >> + * Note that checking QLIST_EMPTY(&bs->children) is also >> done when >> + * the cache is queried above. Technically, we do not >> need to check >> + * it here; the worst that can happen is that we fill >> the cache for >> + * non-protocol nodes, and then it is never used. >> However, filling >> + * the cache requires an RCU update, so double check >> here to avoid >> + * such an update if possible. >> + */ >> + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && >> + QLIST_EMPTY(&bs->children)) >> + { >> + bdrv_bsc_fill(bs, aligned_offset, *pnum); >> + } >> + } >> } else { >> /* Default code for filters */ >> @@ -2997,6 +3049,9 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild >> *child, int64_t offset, >> return 0; >> } >> + /* Invalidate the cached block-status data range if this >> discard overlaps */ >> + bdrv_bsc_invalidate_range(bs, offset, bytes); >> + >> /* Discard is advisory, but some devices track and coalesce >> * unaligned requests, so we must pass everything down rather than >> * round here. Still, most devices will just silently ignore >> > >
Am 23.06.2021 um 17:01 hat Max Reitz geschrieben: > As we have attempted before > (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, > "file-posix: Cache lseek result for data regions"; > https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, > "file-posix: Cache next hole"), this patch seeks to reduce the number of > SEEK_DATA/HOLE operations the file-posix driver has to perform. The > main difference is that this time it is implemented as part of the > general block layer code. > > The problem we face is that on some filesystems or in some > circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the > implementation is outside of qemu, there is little we can do about its > performance. > > We have already introduced the want_zero parameter to > bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls > unless we really want zero information; but sometimes we do want that > information, because for files that consist largely of zero areas, > special-casing those areas can give large performance boosts. So the > real problem is with files that consist largely of data, so that > inquiring the block status does not gain us much performance, but where > such an inquiry itself takes a lot of time. > > To address this, we want to cache data regions. Most of the time, when > bad performance is reported, it is in places where the image is iterated > over from start to end (qemu-img convert or the mirror job), so a simple > yet effective solution is to cache only the current data region. > > (Note that only caching data regions but not zero regions means that > returning false information from the cache is not catastrophic: Treating > zeroes as data is fine. While we try to invalidate the cache on zero > writes and discards, such incongruences may still occur when there are > other processes writing to the image.) > > We only use the cache for nodes without children (i.e. protocol nodes), > because that is where the problem is: Drivers that rely on block-status > implementations outside of qemu (e.g. SEEK_DATA/HOLE). > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 > Signed-off-by: Max Reitz <mreitz@redhat.com> Since you indicated that you'll respin the patch, I'll add my minor comments: > @@ -2442,9 +2445,58 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs, > aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset; > > if (bs->drv->bdrv_co_block_status) { > - ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, > - aligned_bytes, pnum, &local_map, > - &local_file); > + bool from_cache = false; > + > + /* > + * Use the block-status cache only for protocol nodes: Format > + * drivers are generally quick to inquire the status, but protocol > + * drivers often need to get information from outside of qemu, so > + * we do not have control over the actual implementation. There > + * have been cases where inquiring the status took an unreasonably > + * long time, and we can do nothing in qemu to fix it. > + * This is especially problematic for images with large data areas, > + * because finding the few holes in them and giving them special > + * treatment does not gain much performance. Therefore, we try to > + * cache the last-identified data region. > + * > + * Second, limiting ourselves to protocol nodes allows us to assume > + * the block status for data regions to be DATA | OFFSET_VALID, and > + * that the host offset is the same as the guest offset. > + * > + * Note that it is possible that external writers zero parts of > + * the cached regions without the cache being invalidated, and so > + * we may report zeroes as data. This is not catastrophic, > + * however, because reporting zeroes as data is fine. > + */ > + if (QLIST_EMPTY(&bs->children)) { > + if (bdrv_bsc_is_data(bs, aligned_offset, pnum)) { > + ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID; > + local_file = bs; > + local_map = aligned_offset; > + > + from_cache = true; > + } > + } > + > + if (!from_cache) { Is having a separate variable from_cache really useful? This looks like it could just be: if (QLIST_EMPTY() && bdrv_bsc_is_data()) { // The code above } else { // The code below } > + ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, > + aligned_bytes, pnum, &local_map, > + &local_file); > + > + /* > + * Note that checking QLIST_EMPTY(&bs->children) is also done when > + * the cache is queried above. Technically, we do not need to check > + * it here; the worst that can happen is that we fill the cache for > + * non-protocol nodes, and then it is never used. However, filling > + * the cache requires an RCU update, so double check here to avoid > + * such an update if possible. > + */ > + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && > + QLIST_EMPTY(&bs->children)) > + { Would it be worth asserting that local_map == aligned_offset, because otherwise with a buggy protocol driver, the result from the cache could be different from the first call without us noticing? > + bdrv_bsc_fill(bs, aligned_offset, *pnum); > + } > + } > } else { > /* Default code for filters */ Kevin
On 06.07.21 19:04, Kevin Wolf wrote: > Am 23.06.2021 um 17:01 hat Max Reitz geschrieben: >> As we have attempted before >> (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, >> "file-posix: Cache lseek result for data regions"; >> https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, >> "file-posix: Cache next hole"), this patch seeks to reduce the number of >> SEEK_DATA/HOLE operations the file-posix driver has to perform. The >> main difference is that this time it is implemented as part of the >> general block layer code. >> >> The problem we face is that on some filesystems or in some >> circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the >> implementation is outside of qemu, there is little we can do about its >> performance. >> >> We have already introduced the want_zero parameter to >> bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls >> unless we really want zero information; but sometimes we do want that >> information, because for files that consist largely of zero areas, >> special-casing those areas can give large performance boosts. So the >> real problem is with files that consist largely of data, so that >> inquiring the block status does not gain us much performance, but where >> such an inquiry itself takes a lot of time. >> >> To address this, we want to cache data regions. Most of the time, when >> bad performance is reported, it is in places where the image is iterated >> over from start to end (qemu-img convert or the mirror job), so a simple >> yet effective solution is to cache only the current data region. >> >> (Note that only caching data regions but not zero regions means that >> returning false information from the cache is not catastrophic: Treating >> zeroes as data is fine. While we try to invalidate the cache on zero >> writes and discards, such incongruences may still occur when there are >> other processes writing to the image.) >> >> We only use the cache for nodes without children (i.e. protocol nodes), >> because that is where the problem is: Drivers that rely on block-status >> implementations outside of qemu (e.g. SEEK_DATA/HOLE). >> >> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 >> Signed-off-by: Max Reitz <mreitz@redhat.com> > Since you indicated that you'll respin the patch, I'll add my minor > comments: > >> @@ -2442,9 +2445,58 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs, >> aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset; >> >> if (bs->drv->bdrv_co_block_status) { >> - ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, >> - aligned_bytes, pnum, &local_map, >> - &local_file); >> + bool from_cache = false; >> + >> + /* >> + * Use the block-status cache only for protocol nodes: Format >> + * drivers are generally quick to inquire the status, but protocol >> + * drivers often need to get information from outside of qemu, so >> + * we do not have control over the actual implementation. There >> + * have been cases where inquiring the status took an unreasonably >> + * long time, and we can do nothing in qemu to fix it. >> + * This is especially problematic for images with large data areas, >> + * because finding the few holes in them and giving them special >> + * treatment does not gain much performance. Therefore, we try to >> + * cache the last-identified data region. >> + * >> + * Second, limiting ourselves to protocol nodes allows us to assume >> + * the block status for data regions to be DATA | OFFSET_VALID, and >> + * that the host offset is the same as the guest offset. >> + * >> + * Note that it is possible that external writers zero parts of >> + * the cached regions without the cache being invalidated, and so >> + * we may report zeroes as data. This is not catastrophic, >> + * however, because reporting zeroes as data is fine. >> + */ >> + if (QLIST_EMPTY(&bs->children)) { >> + if (bdrv_bsc_is_data(bs, aligned_offset, pnum)) { >> + ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID; >> + local_file = bs; >> + local_map = aligned_offset; >> + >> + from_cache = true; >> + } >> + } >> + >> + if (!from_cache) { > Is having a separate variable from_cache really useful? This looks like > it could just be: > > if (QLIST_EMPTY() && bdrv_bsc_is_data()) { > // The code above > } else { > // The code below > } Oh, yes. (I guess this was mainly an artifact from v1 where there was a mutex around the bdrv_bsc_is_data() block. Now it’s better to just roll both conditions into one, yes.) >> + ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, >> + aligned_bytes, pnum, &local_map, >> + &local_file); >> + >> + /* >> + * Note that checking QLIST_EMPTY(&bs->children) is also done when >> + * the cache is queried above. Technically, we do not need to check >> + * it here; the worst that can happen is that we fill the cache for >> + * non-protocol nodes, and then it is never used. However, filling >> + * the cache requires an RCU update, so double check here to avoid >> + * such an update if possible. >> + */ >> + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && >> + QLIST_EMPTY(&bs->children)) >> + { > Would it be worth asserting that local_map == aligned_offset, because > otherwise with a buggy protocol driver, the result from the cache could > be different from the first call without us noticing? I think it would be indeed. Max >> + bdrv_bsc_fill(bs, aligned_offset, *pnum); >> + } >> + } >> } else { >> /* Default code for filters */ > Kevin >
diff --git a/include/block/block_int.h b/include/block/block_int.h index a8f9598102..fcb599dd1c 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -832,6 +832,22 @@ struct BdrvChild { QLIST_ENTRY(BdrvChild) next_parent; }; +/* + * Allows bdrv_co_block_status() to cache one data region for a + * protocol node. + * + * @valid: Whether the cache is valid (should be accessed with atomic + * functions so this can be reset by RCU readers) + * @data_start: Offset where we know (or strongly assume) is data + * @data_end: Offset where the data region ends (which is not necessarily + * the start of a zeroed region) + */ +typedef struct BdrvBlockStatusCache { + bool valid; + int64_t data_start; + int64_t data_end; +} BdrvBlockStatusCache; + struct BlockDriverState { /* Protected by big QEMU lock or read-only after opening. No special * locking needed during I/O... @@ -997,6 +1013,11 @@ struct BlockDriverState { /* BdrvChild links to this node may never be frozen */ bool never_freeze; + + /* Lock for block-status cache RCU writers */ + CoMutex bsc_modify_lock; + /* Always non-NULL, but must only be dereferenced under an RCU read guard */ + BdrvBlockStatusCache *block_status_cache; }; struct BlockBackendRootState { @@ -1422,4 +1443,30 @@ static inline BlockDriverState *bdrv_primary_bs(BlockDriverState *bs) */ void bdrv_drain_all_end_quiesce(BlockDriverState *bs); +/** + * Check whether the given offset is in the cached block-status data + * region. + * + * If it is, and @pnum is not NULL, *pnum is set to + * `bsc.data_end - offset`, i.e. how many bytes, starting from + * @offset, are data (according to the cache). + * Otherwise, *pnum is not touched. + */ +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum); + +/** + * If [offset, offset + bytes) overlaps with the currently cached + * block-status region, invalidate the cache. + * + * (To be used by I/O paths that cause data regions to be zero or + * holes.) + */ +void bdrv_bsc_invalidate_range(BlockDriverState *bs, + int64_t offset, int64_t bytes); + +/** + * Mark the range [offset, offset + bytes) as a data region. + */ +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes); + #endif /* BLOCK_INT_H */ diff --git a/block.c b/block.c index 3f456892d0..9ab9459f7a 100644 --- a/block.c +++ b/block.c @@ -49,6 +49,8 @@ #include "qemu/timer.h" #include "qemu/cutils.h" #include "qemu/id.h" +#include "qemu/range.h" +#include "qemu/rcu.h" #include "block/coroutines.h" #ifdef CONFIG_BSD @@ -398,6 +400,9 @@ BlockDriverState *bdrv_new(void) qemu_co_queue_init(&bs->flush_queue); + qemu_co_mutex_init(&bs->bsc_modify_lock); + bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1); + for (i = 0; i < bdrv_drain_all_count; i++) { bdrv_drained_begin(bs); } @@ -4635,6 +4640,8 @@ static void bdrv_close(BlockDriverState *bs) bs->explicit_options = NULL; qobject_unref(bs->full_open_options); bs->full_open_options = NULL; + g_free(bs->block_status_cache); + bs->block_status_cache = NULL; bdrv_release_named_dirty_bitmaps(bs); assert(QLIST_EMPTY(&bs->dirty_bitmaps)); @@ -7590,3 +7597,80 @@ BlockDriverState *bdrv_backing_chain_next(BlockDriverState *bs) { return bdrv_skip_filters(bdrv_cow_bs(bdrv_skip_filters(bs))); } + +/** + * Check whether [offset, offset + bytes) overlaps with the cached + * block-status data region. + * + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`, + * which is what bdrv_bsc_is_data()'s interface needs. + * Otherwise, *pnum is not touched. + */ +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs, + int64_t offset, int64_t bytes, + int64_t *pnum) +{ + BdrvBlockStatusCache *bsc = bs->block_status_cache; + bool overlaps; + + overlaps = + qatomic_read(&bsc->valid) && + ranges_overlap(offset, bytes, bsc->data_start, + bsc->data_end - bsc->data_start); + + if (overlaps && pnum) { + *pnum = bsc->data_end - offset; + } + + return overlaps; +} + +/** + * See block_int.h for this function's documentation. + */ +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum) +{ + bool overlaps; + + WITH_RCU_READ_LOCK_GUARD() { + overlaps = bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum); + } + + return overlaps; +} + +/** + * See block_int.h for this function's documentation. + */ +void bdrv_bsc_invalidate_range(BlockDriverState *bs, + int64_t offset, int64_t bytes) +{ + WITH_RCU_READ_LOCK_GUARD() { + if (bdrv_bsc_range_overlaps_locked(bs, offset, bytes, NULL)) { + qatomic_set(&bs->block_status_cache->valid, false); + } + } +} + +/** + * See block_int.h for this function's documentation. + */ +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes) +{ + BdrvBlockStatusCache *new_bsc = g_new(BdrvBlockStatusCache, 1); + BdrvBlockStatusCache *old_bsc; + + *new_bsc = (BdrvBlockStatusCache) { + .valid = true, + .data_start = offset, + .data_end = offset + bytes, + }; + + WITH_QEMU_LOCK_GUARD(&bs->bsc_modify_lock) { + old_bsc = bs->block_status_cache; + qatomic_rcu_set(&bs->block_status_cache, new_bsc); + synchronize_rcu(); + } + + g_free(old_bsc); +} diff --git a/block/io.c b/block/io.c index 323854d063..85fa449bf9 100644 --- a/block/io.c +++ b/block/io.c @@ -1878,6 +1878,9 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, return -ENOTSUP; } + /* Invalidate the cached block-status data range if this write overlaps */ + bdrv_bsc_invalidate_range(bs, offset, bytes); + assert(alignment % bs->bl.request_alignment == 0); head = offset % alignment; tail = (offset + bytes) % alignment; @@ -2442,9 +2445,58 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs, aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset; if (bs->drv->bdrv_co_block_status) { - ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, - aligned_bytes, pnum, &local_map, - &local_file); + bool from_cache = false; + + /* + * Use the block-status cache only for protocol nodes: Format + * drivers are generally quick to inquire the status, but protocol + * drivers often need to get information from outside of qemu, so + * we do not have control over the actual implementation. There + * have been cases where inquiring the status took an unreasonably + * long time, and we can do nothing in qemu to fix it. + * This is especially problematic for images with large data areas, + * because finding the few holes in them and giving them special + * treatment does not gain much performance. Therefore, we try to + * cache the last-identified data region. + * + * Second, limiting ourselves to protocol nodes allows us to assume + * the block status for data regions to be DATA | OFFSET_VALID, and + * that the host offset is the same as the guest offset. + * + * Note that it is possible that external writers zero parts of + * the cached regions without the cache being invalidated, and so + * we may report zeroes as data. This is not catastrophic, + * however, because reporting zeroes as data is fine. + */ + if (QLIST_EMPTY(&bs->children)) { + if (bdrv_bsc_is_data(bs, aligned_offset, pnum)) { + ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID; + local_file = bs; + local_map = aligned_offset; + + from_cache = true; + } + } + + if (!from_cache) { + ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, + aligned_bytes, pnum, &local_map, + &local_file); + + /* + * Note that checking QLIST_EMPTY(&bs->children) is also done when + * the cache is queried above. Technically, we do not need to check + * it here; the worst that can happen is that we fill the cache for + * non-protocol nodes, and then it is never used. However, filling + * the cache requires an RCU update, so double check here to avoid + * such an update if possible. + */ + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && + QLIST_EMPTY(&bs->children)) + { + bdrv_bsc_fill(bs, aligned_offset, *pnum); + } + } } else { /* Default code for filters */ @@ -2997,6 +3049,9 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset, return 0; } + /* Invalidate the cached block-status data range if this discard overlaps */ + bdrv_bsc_invalidate_range(bs, offset, bytes); + /* Discard is advisory, but some devices track and coalesce * unaligned requests, so we must pass everything down rather than * round here. Still, most devices will just silently ignore
As we have attempted before (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, "file-posix: Cache lseek result for data regions"; https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, "file-posix: Cache next hole"), this patch seeks to reduce the number of SEEK_DATA/HOLE operations the file-posix driver has to perform. The main difference is that this time it is implemented as part of the general block layer code. The problem we face is that on some filesystems or in some circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the implementation is outside of qemu, there is little we can do about its performance. We have already introduced the want_zero parameter to bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls unless we really want zero information; but sometimes we do want that information, because for files that consist largely of zero areas, special-casing those areas can give large performance boosts. So the real problem is with files that consist largely of data, so that inquiring the block status does not gain us much performance, but where such an inquiry itself takes a lot of time. To address this, we want to cache data regions. Most of the time, when bad performance is reported, it is in places where the image is iterated over from start to end (qemu-img convert or the mirror job), so a simple yet effective solution is to cache only the current data region. (Note that only caching data regions but not zero regions means that returning false information from the cache is not catastrophic: Treating zeroes as data is fine. While we try to invalidate the cache on zero writes and discards, such incongruences may still occur when there are other processes writing to the image.) We only use the cache for nodes without children (i.e. protocol nodes), because that is where the problem is: Drivers that rely on block-status implementations outside of qemu (e.g. SEEK_DATA/HOLE). Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 Signed-off-by: Max Reitz <mreitz@redhat.com> --- include/block/block_int.h | 47 ++++++++++++++++++++++ block.c | 84 +++++++++++++++++++++++++++++++++++++++ block/io.c | 61 ++++++++++++++++++++++++++-- 3 files changed, 189 insertions(+), 3 deletions(-)