| Message ID | 20241114152743.2381672-12-axboe@kernel.dk (mailing list archive) |
|---|---|
| State | Not Applicable, archived |
| Headers | show |
| Series | Uncached buffered IO | expand |
On 2024/11/14 23:25, Jens Axboe wrote: > If RWF_UNCACHED is set for a write, mark new folios being written with > uncached. This is done by passing in the fact that it's an uncached write > through the folio pointer. We can only get there when IOCB_UNCACHED was > allowed, which can only happen if the file system opts in. Opting in means > they need to check for the LSB in the folio pointer to know if it's an > uncached write or not. If it is, then FGP_UNCACHED should be used if > creating new folios is necessary. > > Uncached writes will drop any folios they create upon writeback > completion, but leave folios that may exist in that range alone. Since > ->write_begin() doesn't currently take any flags, and to avoid needing > to change the callback kernel wide, use the foliop being passed in to > ->write_begin() to signal if this is an uncached write or not. File > systems can then use that to mark newly created folios as uncached. > > This provides similar benefits to using RWF_UNCACHED with reads. Testing > buffered writes on 32 files: > > writing bs 65536, uncached 0 > 1s: 196035MB/sec > 2s: 132308MB/sec > 3s: 132438MB/sec > 4s: 116528MB/sec > 5s: 103898MB/sec > 6s: 108893MB/sec > 7s: 99678MB/sec > 8s: 106545MB/sec > 9s: 106826MB/sec > 10s: 101544MB/sec > 11s: 111044MB/sec > 12s: 124257MB/sec > 13s: 116031MB/sec > 14s: 114540MB/sec > 15s: 115011MB/sec > 16s: 115260MB/sec > 17s: 116068MB/sec > 18s: 116096MB/sec > > where it's quite obvious where the page cache filled, and performance > dropped from to about half of where it started, settling in at around > 115GB/sec. Meanwhile, 32 kswapds were running full steam trying to > reclaim pages. > > Running the same test with uncached buffered writes: > > writing bs 65536, uncached 1 > 1s: 198974MB/sec > 2s: 189618MB/sec > 3s: 193601MB/sec > 4s: 188582MB/sec > 5s: 193487MB/sec > 6s: 188341MB/sec > 7s: 194325MB/sec > 8s: 188114MB/sec > 9s: 192740MB/sec > 10s: 189206MB/sec > 11s: 193442MB/sec > 12s: 189659MB/sec > 13s: 191732MB/sec > 14s: 190701MB/sec > 15s: 191789MB/sec > 16s: 191259MB/sec > 17s: 190613MB/sec > 18s: 191951MB/sec > > and the behavior is fully predictable, performing the same throughout > even after the page cache would otherwise have fully filled with dirty > data. It's also about 65% faster, and using half the CPU of the system > compared to the normal buffered write. > > Signed-off-by: Jens Axboe <axboe@kernel.dk> > --- > include/linux/fs.h | 5 +++++ > include/linux/pagemap.h | 9 +++++++++ > mm/filemap.c | 12 +++++++++++- > 3 files changed, 25 insertions(+), 1 deletion(-) > > diff --git a/include/linux/fs.h b/include/linux/fs.h > index 45510d0b8de0..122ae821989f 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -2877,6 +2877,11 @@ static inline ssize_t generic_write_sync(struct kiocb *iocb, ssize_t count) > (iocb->ki_flags & IOCB_SYNC) ? 0 : 1); > if (ret) > return ret; > + } else if (iocb->ki_flags & IOCB_UNCACHED) { > + struct address_space *mapping = iocb->ki_filp->f_mapping; > + > + filemap_fdatawrite_range_kick(mapping, iocb->ki_pos, > + iocb->ki_pos + count); > } > Hi Jens, The filemap_fdatawrite_range_kick() helper function is not added until the next patch, so you should swap the order of patch 10 and patch 11. Regards, Baokun > return count; > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index d55bf995bd9e..cc02518d338d 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -14,6 +14,7 @@ > #include <linux/gfp.h> > #include <linux/bitops.h> > #include <linux/hardirq.h> /* for in_interrupt() */ > +#include <linux/writeback.h> > #include <linux/hugetlb_inline.h> > > struct folio_batch; > @@ -70,6 +71,14 @@ static inline int filemap_write_and_wait(struct address_space *mapping) > return filemap_write_and_wait_range(mapping, 0, LLONG_MAX); > } > > +/* > + * Value passed in to ->write_begin() if IOCB_UNCACHED is set for the write, > + * and the ->write_begin() handler on a file system supporting FOP_UNCACHED > + * must check for this and pass FGP_UNCACHED for folio creation. > + */ > +#define foliop_uncached ((struct folio *) 0xfee1c001) > +#define foliop_is_uncached(foliop) (*(foliop) == foliop_uncached) > + > /** > * filemap_set_wb_err - set a writeback error on an address_space > * @mapping: mapping in which to set writeback error > diff --git a/mm/filemap.c b/mm/filemap.c > index 13815194ed8a..297cb53332ff 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -4076,7 +4076,7 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i) > ssize_t written = 0; > > do { > - struct folio *folio; > + struct folio *folio = NULL; > size_t offset; /* Offset into folio */ > size_t bytes; /* Bytes to write to folio */ > size_t copied; /* Bytes copied from user */ > @@ -4104,6 +4104,16 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i) > break; > } > > + /* > + * If IOCB_UNCACHED is set here, we now the file system > + * supports it. And hence it'll know to check folip for being > + * set to this magic value. If so, it's an uncached write. > + * Whenever ->write_begin() changes prototypes again, this > + * can go away and just pass iocb or iocb flags. > + */ > + if (iocb->ki_flags & IOCB_UNCACHED) > + folio = foliop_uncached; > + > status = a_ops->write_begin(file, mapping, pos, bytes, > &folio, &fsdata); > if (unlikely(status < 0))
On 11/18/24 1:42 AM, Baokun Li wrote: >> diff --git a/include/linux/fs.h b/include/linux/fs.h >> index 45510d0b8de0..122ae821989f 100644 >> --- a/include/linux/fs.h >> +++ b/include/linux/fs.h >> @@ -2877,6 +2877,11 @@ static inline ssize_t generic_write_sync(struct kiocb *iocb, ssize_t count) >> (iocb->ki_flags & IOCB_SYNC) ? 0 : 1); >> if (ret) >> return ret; >> + } else if (iocb->ki_flags & IOCB_UNCACHED) { >> + struct address_space *mapping = iocb->ki_filp->f_mapping; >> + >> + filemap_fdatawrite_range_kick(mapping, iocb->ki_pos, >> + iocb->ki_pos + count); >> } >> > > Hi Jens, > > The filemap_fdatawrite_range_kick() helper function is not added until > the next patch, so you should swap the order of patch 10 and patch 11. Ah thanks, not sure how I missed that. I'll swap them for the next posting, and also do a basic bisection test just to ensure I did't do more of those...
diff --git a/include/linux/fs.h b/include/linux/fs.h index 45510d0b8de0..122ae821989f 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2877,6 +2877,11 @@ static inline ssize_t generic_write_sync(struct kiocb *iocb, ssize_t count) (iocb->ki_flags & IOCB_SYNC) ? 0 : 1); if (ret) return ret; + } else if (iocb->ki_flags & IOCB_UNCACHED) { + struct address_space *mapping = iocb->ki_filp->f_mapping; + + filemap_fdatawrite_range_kick(mapping, iocb->ki_pos, + iocb->ki_pos + count); } return count; diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index d55bf995bd9e..cc02518d338d 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -14,6 +14,7 @@ #include <linux/gfp.h> #include <linux/bitops.h> #include <linux/hardirq.h> /* for in_interrupt() */ +#include <linux/writeback.h> #include <linux/hugetlb_inline.h> struct folio_batch; @@ -70,6 +71,14 @@ static inline int filemap_write_and_wait(struct address_space *mapping) return filemap_write_and_wait_range(mapping, 0, LLONG_MAX); } +/* + * Value passed in to ->write_begin() if IOCB_UNCACHED is set for the write, + * and the ->write_begin() handler on a file system supporting FOP_UNCACHED + * must check for this and pass FGP_UNCACHED for folio creation. + */ +#define foliop_uncached ((struct folio *) 0xfee1c001) +#define foliop_is_uncached(foliop) (*(foliop) == foliop_uncached) + /** * filemap_set_wb_err - set a writeback error on an address_space * @mapping: mapping in which to set writeback error diff --git a/mm/filemap.c b/mm/filemap.c index 13815194ed8a..297cb53332ff 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -4076,7 +4076,7 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i) ssize_t written = 0; do { - struct folio *folio; + struct folio *folio = NULL; size_t offset; /* Offset into folio */ size_t bytes; /* Bytes to write to folio */ size_t copied; /* Bytes copied from user */ @@ -4104,6 +4104,16 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i) break; } + /* + * If IOCB_UNCACHED is set here, we now the file system + * supports it. And hence it'll know to check folip for being + * set to this magic value. If so, it's an uncached write. + * Whenever ->write_begin() changes prototypes again, this + * can go away and just pass iocb or iocb flags. + */ + if (iocb->ki_flags & IOCB_UNCACHED) + folio = foliop_uncached; + status = a_ops->write_begin(file, mapping, pos, bytes, &folio, &fsdata); if (unlikely(status < 0))
If RWF_UNCACHED is set for a write, mark new folios being written with uncached. This is done by passing in the fact that it's an uncached write through the folio pointer. We can only get there when IOCB_UNCACHED was allowed, which can only happen if the file system opts in. Opting in means they need to check for the LSB in the folio pointer to know if it's an uncached write or not. If it is, then FGP_UNCACHED should be used if creating new folios is necessary. Uncached writes will drop any folios they create upon writeback completion, but leave folios that may exist in that range alone. Since ->write_begin() doesn't currently take any flags, and to avoid needing to change the callback kernel wide, use the foliop being passed in to ->write_begin() to signal if this is an uncached write or not. File systems can then use that to mark newly created folios as uncached. This provides similar benefits to using RWF_UNCACHED with reads. Testing buffered writes on 32 files: writing bs 65536, uncached 0 1s: 196035MB/sec 2s: 132308MB/sec 3s: 132438MB/sec 4s: 116528MB/sec 5s: 103898MB/sec 6s: 108893MB/sec 7s: 99678MB/sec 8s: 106545MB/sec 9s: 106826MB/sec 10s: 101544MB/sec 11s: 111044MB/sec 12s: 124257MB/sec 13s: 116031MB/sec 14s: 114540MB/sec 15s: 115011MB/sec 16s: 115260MB/sec 17s: 116068MB/sec 18s: 116096MB/sec where it's quite obvious where the page cache filled, and performance dropped from to about half of where it started, settling in at around 115GB/sec. Meanwhile, 32 kswapds were running full steam trying to reclaim pages. Running the same test with uncached buffered writes: writing bs 65536, uncached 1 1s: 198974MB/sec 2s: 189618MB/sec 3s: 193601MB/sec 4s: 188582MB/sec 5s: 193487MB/sec 6s: 188341MB/sec 7s: 194325MB/sec 8s: 188114MB/sec 9s: 192740MB/sec 10s: 189206MB/sec 11s: 193442MB/sec 12s: 189659MB/sec 13s: 191732MB/sec 14s: 190701MB/sec 15s: 191789MB/sec 16s: 191259MB/sec 17s: 190613MB/sec 18s: 191951MB/sec and the behavior is fully predictable, performing the same throughout even after the page cache would otherwise have fully filled with dirty data. It's also about 65% faster, and using half the CPU of the system compared to the normal buffered write. Signed-off-by: Jens Axboe <axboe@kernel.dk> --- include/linux/fs.h | 5 +++++ include/linux/pagemap.h | 9 +++++++++ mm/filemap.c | 12 +++++++++++- 3 files changed, 25 insertions(+), 1 deletion(-)