| Message ID | 1479410660-31408-1-git-send-email-dianders@chromium.org (mailing list archive) |
|---|---|
| State | Accepted, archived |
| Delegated to: | Mike Snitzer |
| Headers | show |
On Thu, Nov 17 2016 at 3:44pm -0500, Doug Anderson <dianders@chromium.org> wrote: > Hi, > > On Thu, Nov 17, 2016 at 12:28 PM, Mike Snitzer <snitzer@redhat.com> wrote: > > On Thu, Nov 17 2016 at 2:24pm -0500, > > Douglas Anderson <dianders@chromium.org> wrote: > > > >> We've seen in-field reports showing _lots_ (18 in one case, 41 in > >> another) of tasks all sitting there blocked on: > >> > >> mutex_lock+0x4c/0x68 > >> dm_bufio_shrink_count+0x38/0x78 > >> shrink_slab.part.54.constprop.65+0x100/0x464 > >> shrink_zone+0xa8/0x198 > >> > >> In the two cases analyzed, we see one task that looks like this: > >> > >> Workqueue: kverityd verity_prefetch_io > >> > >> __switch_to+0x9c/0xa8 > >> __schedule+0x440/0x6d8 > >> schedule+0x94/0xb4 > >> schedule_timeout+0x204/0x27c > >> schedule_timeout_uninterruptible+0x44/0x50 > >> wait_iff_congested+0x9c/0x1f0 > >> shrink_inactive_list+0x3a0/0x4cc > >> shrink_lruvec+0x418/0x5cc > >> shrink_zone+0x88/0x198 > >> try_to_free_pages+0x51c/0x588 > >> __alloc_pages_nodemask+0x648/0xa88 > >> __get_free_pages+0x34/0x7c > >> alloc_buffer+0xa4/0x144 > >> __bufio_new+0x84/0x278 > >> dm_bufio_prefetch+0x9c/0x154 > >> verity_prefetch_io+0xe8/0x10c > >> process_one_work+0x240/0x424 > >> worker_thread+0x2fc/0x424 > >> kthread+0x10c/0x114 > >> > >> ...and that looks to be the one holding the mutex. > >> > >> The problem has been reproduced on fairly easily: > >> 0. Be running Chrome OS w/ verity enabled on the root filesystem > >> 1. Pick test patch: http://crosreview.com/412360 > >> 2. Install launchBalloons.sh and balloon.arm from > >> http://crbug.com/468342 > >> ...that's just a memory stress test app. > >> 3. On a 4GB rk3399 machine, run > >> nice ./launchBalloons.sh 4 900 100000 > >> ...that tries to eat 4 * 900 MB of memory and keep accessing. > >> 4. Login to the Chrome web browser and restore many tabs > >> > >> With that, I've seen printouts like: > >> DOUG: long bufio 90758 ms > >> ...and stack trace always show's we're in dm_bufio_prefetch(). > >> > >> The problem is that we try to allocate memory with GFP_NOIO while > >> we're holding the dm_bufio lock. Instead we should be using > >> GFP_NOWAIT. Using GFP_NOIO can cause us to sleep while holding the > >> lock and that causes the above problems. > >> > >> The current behavior explained by David Rientjes: > >> > >> It will still try reclaim initially because __GFP_WAIT (or > >> __GFP_KSWAPD_RECLAIM) is set by GFP_NOIO. This is the cause of > >> contention on dm_bufio_lock() that the thread holds. You want to > >> pass GFP_NOWAIT instead of GFP_NOIO to alloc_buffer() when holding a > >> mutex that can be contended by a concurrent slab shrinker (if > >> count_objects didn't use a trylock, this pattern would trivially > >> deadlock). > >> > >> Suggested-by: David Rientjes <rientjes@google.com> > >> Signed-off-by: Douglas Anderson <dianders@chromium.org> > >> --- > >> Note that this change was developed and tested against the Chrome OS > >> 4.4 kernel tree, not mainline. Due to slight differences in verity > >> between mainline and Chrome OS it became too difficult to reproduce my > >> testing setup on mainline. This patch still seems correct and > >> relevant to upstream, so I'm posting it. If this is not acceptible to > >> you then please ignore this patch. > >> > >> Also note that when I tested the Chrome OS 3.14 kernel tree I couldn't > >> reproduce the long delays described in the patch. Presumably > >> something changed in either the kernel config or the memory management > >> code between the two kernel versions that made this crop up. In a > >> similar vein, it is possible that problems described in this patch are > >> no longer reproducible upstream. However, the arguments made in this > >> patch (that we don't want to block while holding the mutex) still > >> apply so I think the patch may still have merit. > >> > >> drivers/md/dm-bufio.c | 6 ++++-- > >> 1 file changed, 4 insertions(+), 2 deletions(-) > >> > >> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c > >> index b3ba142e59a4..3c767399cc59 100644 > >> --- a/drivers/md/dm-bufio.c > >> +++ b/drivers/md/dm-bufio.c > >> @@ -827,7 +827,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client > >> * dm-bufio is resistant to allocation failures (it just keeps > >> * one buffer reserved in cases all the allocations fail). > >> * So set flags to not try too hard: > >> - * GFP_NOIO: don't recurse into the I/O layer > >> + * GFP_NOWAIT: don't wait; if we need to sleep we'll release our > >> + * mutex and wait ourselves. > >> * __GFP_NORETRY: don't retry and rather return failure > >> * __GFP_NOMEMALLOC: don't use emergency reserves > >> * __GFP_NOWARN: don't print a warning in case of failure > >> @@ -837,7 +838,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client > >> */ > >> while (1) { > >> if (dm_bufio_cache_size_latch != 1) { > >> - b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); > >> + b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | > >> + __GFP_NOMEMALLOC | __GFP_NOWARN); > >> if (b) > >> return b; > >> } > >> -- > >> 2.8.0.rc3.226.g39d4020 > >> > > > > I have one report of a very low-memory system hitting issues with bufio > > (in the context of DM-thinp, due to bufio shrinker) but nothing > > implicating alloc_buffer(). > > > > In any case, I'm fine with your patch given that we'll just retry. BUT > > spinning in __alloc_buffer_wait_no_callback() doesn't really change the > > fact that you're starved for memory. It just makes this less visible > > right? Meaning that you won't see hung task timeouts? Or were you > > seeing these tasks manifest this back-pressure through other means? > > It actually significantly increases responsiveness of the system while > in this state, so it makes a real difference. I believe it actually > changes behavior because it (at least) unblocks kswapd. In the bug > report I analyzed, I saw: > > kswapd0 D ffffffc000204fd8 0 72 2 0x00000000 > Call trace: > [<ffffffc000204fd8>] __switch_to+0x9c/0xa8 > [<ffffffc00090b794>] __schedule+0x440/0x6d8 > [<ffffffc00090bac0>] schedule+0x94/0xb4 > [<ffffffc00090be44>] schedule_preempt_disabled+0x28/0x44 > [<ffffffc00090d900>] __mutex_lock_slowpath+0x120/0x1ac > [<ffffffc00090d9d8>] mutex_lock+0x4c/0x68 > [<ffffffc000708e7c>] dm_bufio_shrink_count+0x38/0x78 > [<ffffffc00030b268>] shrink_slab.part.54.constprop.65+0x100/0x464 > [<ffffffc00030dbd8>] shrink_zone+0xa8/0x198 > [<ffffffc00030e578>] balance_pgdat+0x328/0x508 > [<ffffffc00030eb7c>] kswapd+0x424/0x51c > [<ffffffc00023f06c>] kthread+0x10c/0x114 > [<ffffffc000203dd0>] ret_from_fork+0x10/0x40 > > I'm not an expert, but I believe that blocking swapd isn't a super > great idea and that if we unblock it (like my patch will) then that > can help alleviate memory pressure. OK, thanks for clarifying. I'll get it staged for 4.10 this week. -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel
On Thu, Nov 17, 2016 at 11:24:20AM -0800, Douglas Anderson wrote: > We've seen in-field reports showing _lots_ (18 in one case, 41 in > another) of tasks all sitting there blocked on: > > mutex_lock+0x4c/0x68 > dm_bufio_shrink_count+0x38/0x78 > shrink_slab.part.54.constprop.65+0x100/0x464 > shrink_zone+0xa8/0x198 > > In the two cases analyzed, we see one task that looks like this: > > Workqueue: kverityd verity_prefetch_io > > __switch_to+0x9c/0xa8 > __schedule+0x440/0x6d8 > schedule+0x94/0xb4 > schedule_timeout+0x204/0x27c > schedule_timeout_uninterruptible+0x44/0x50 > wait_iff_congested+0x9c/0x1f0 > shrink_inactive_list+0x3a0/0x4cc > shrink_lruvec+0x418/0x5cc > shrink_zone+0x88/0x198 > try_to_free_pages+0x51c/0x588 > __alloc_pages_nodemask+0x648/0xa88 > __get_free_pages+0x34/0x7c > alloc_buffer+0xa4/0x144 > __bufio_new+0x84/0x278 > dm_bufio_prefetch+0x9c/0x154 > verity_prefetch_io+0xe8/0x10c > process_one_work+0x240/0x424 > worker_thread+0x2fc/0x424 > kthread+0x10c/0x114 > > ...and that looks to be the one holding the mutex. > > The problem has been reproduced on fairly easily: > 0. Be running Chrome OS w/ verity enabled on the root filesystem > 1. Pick test patch: http://crosreview.com/412360 > 2. Install launchBalloons.sh and balloon.arm from > http://crbug.com/468342 > ...that's just a memory stress test app. > 3. On a 4GB rk3399 machine, run > nice ./launchBalloons.sh 4 900 100000 > ...that tries to eat 4 * 900 MB of memory and keep accessing. > 4. Login to the Chrome web browser and restore many tabs > > With that, I've seen printouts like: > DOUG: long bufio 90758 ms > ...and stack trace always show's we're in dm_bufio_prefetch(). > > The problem is that we try to allocate memory with GFP_NOIO while > we're holding the dm_bufio lock. Instead we should be using > GFP_NOWAIT. Using GFP_NOIO can cause us to sleep while holding the > lock and that causes the above problems. > > The current behavior explained by David Rientjes: > > It will still try reclaim initially because __GFP_WAIT (or > __GFP_KSWAPD_RECLAIM) is set by GFP_NOIO. This is the cause of > contention on dm_bufio_lock() that the thread holds. You want to > pass GFP_NOWAIT instead of GFP_NOIO to alloc_buffer() when holding a > mutex that can be contended by a concurrent slab shrinker (if > count_objects didn't use a trylock, this pattern would trivially > deadlock). > > Suggested-by: David Rientjes <rientjes@google.com> > Signed-off-by: Douglas Anderson <dianders@chromium.org> Reviewed-by: Guenter Roeck <linux@roeck-us.net> > --- > Note that this change was developed and tested against the Chrome OS > 4.4 kernel tree, not mainline. Due to slight differences in verity > between mainline and Chrome OS it became too difficult to reproduce my > testing setup on mainline. This patch still seems correct and > relevant to upstream, so I'm posting it. If this is not acceptible to > you then please ignore this patch. > > Also note that when I tested the Chrome OS 3.14 kernel tree I couldn't > reproduce the long delays described in the patch. Presumably > something changed in either the kernel config or the memory management > code between the two kernel versions that made this crop up. In a > similar vein, it is possible that problems described in this patch are > no longer reproducible upstream. However, the arguments made in this > patch (that we don't want to block while holding the mutex) still > apply so I think the patch may still have merit. > > drivers/md/dm-bufio.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c > index b3ba142e59a4..3c767399cc59 100644 > --- a/drivers/md/dm-bufio.c > +++ b/drivers/md/dm-bufio.c > @@ -827,7 +827,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client > * dm-bufio is resistant to allocation failures (it just keeps > * one buffer reserved in cases all the allocations fail). > * So set flags to not try too hard: > - * GFP_NOIO: don't recurse into the I/O layer > + * GFP_NOWAIT: don't wait; if we need to sleep we'll release our > + * mutex and wait ourselves. > * __GFP_NORETRY: don't retry and rather return failure > * __GFP_NOMEMALLOC: don't use emergency reserves > * __GFP_NOWARN: don't print a warning in case of failure > @@ -837,7 +838,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client > */ > while (1) { > if (dm_bufio_cache_size_latch != 1) { > - b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); > + b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | > + __GFP_NOMEMALLOC | __GFP_NOWARN); > if (b) > return b; > } > -- > 2.8.0.rc3.226.g39d4020 > -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel
diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c index b3ba142e59a4..3c767399cc59 100644 --- a/drivers/md/dm-bufio.c +++ b/drivers/md/dm-bufio.c @@ -827,7 +827,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client * dm-bufio is resistant to allocation failures (it just keeps * one buffer reserved in cases all the allocations fail). * So set flags to not try too hard: - * GFP_NOIO: don't recurse into the I/O layer + * GFP_NOWAIT: don't wait; if we need to sleep we'll release our + * mutex and wait ourselves. * __GFP_NORETRY: don't retry and rather return failure * __GFP_NOMEMALLOC: don't use emergency reserves * __GFP_NOWARN: don't print a warning in case of failure @@ -837,7 +838,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client */ while (1) { if (dm_bufio_cache_size_latch != 1) { - b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); + b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | + __GFP_NOMEMALLOC | __GFP_NOWARN); if (b) return b; }
We've seen in-field reports showing _lots_ (18 in one case, 41 in another) of tasks all sitting there blocked on: mutex_lock+0x4c/0x68 dm_bufio_shrink_count+0x38/0x78 shrink_slab.part.54.constprop.65+0x100/0x464 shrink_zone+0xa8/0x198 In the two cases analyzed, we see one task that looks like this: Workqueue: kverityd verity_prefetch_io __switch_to+0x9c/0xa8 __schedule+0x440/0x6d8 schedule+0x94/0xb4 schedule_timeout+0x204/0x27c schedule_timeout_uninterruptible+0x44/0x50 wait_iff_congested+0x9c/0x1f0 shrink_inactive_list+0x3a0/0x4cc shrink_lruvec+0x418/0x5cc shrink_zone+0x88/0x198 try_to_free_pages+0x51c/0x588 __alloc_pages_nodemask+0x648/0xa88 __get_free_pages+0x34/0x7c alloc_buffer+0xa4/0x144 __bufio_new+0x84/0x278 dm_bufio_prefetch+0x9c/0x154 verity_prefetch_io+0xe8/0x10c process_one_work+0x240/0x424 worker_thread+0x2fc/0x424 kthread+0x10c/0x114 ...and that looks to be the one holding the mutex. The problem has been reproduced on fairly easily: 0. Be running Chrome OS w/ verity enabled on the root filesystem 1. Pick test patch: http://crosreview.com/412360 2. Install launchBalloons.sh and balloon.arm from http://crbug.com/468342 ...that's just a memory stress test app. 3. On a 4GB rk3399 machine, run nice ./launchBalloons.sh 4 900 100000 ...that tries to eat 4 * 900 MB of memory and keep accessing. 4. Login to the Chrome web browser and restore many tabs With that, I've seen printouts like: DOUG: long bufio 90758 ms ...and stack trace always show's we're in dm_bufio_prefetch(). The problem is that we try to allocate memory with GFP_NOIO while we're holding the dm_bufio lock. Instead we should be using GFP_NOWAIT. Using GFP_NOIO can cause us to sleep while holding the lock and that causes the above problems. The current behavior explained by David Rientjes: It will still try reclaim initially because __GFP_WAIT (or __GFP_KSWAPD_RECLAIM) is set by GFP_NOIO. This is the cause of contention on dm_bufio_lock() that the thread holds. You want to pass GFP_NOWAIT instead of GFP_NOIO to alloc_buffer() when holding a mutex that can be contended by a concurrent slab shrinker (if count_objects didn't use a trylock, this pattern would trivially deadlock). Suggested-by: David Rientjes <rientjes@google.com> Signed-off-by: Douglas Anderson <dianders@chromium.org> --- Note that this change was developed and tested against the Chrome OS 4.4 kernel tree, not mainline. Due to slight differences in verity between mainline and Chrome OS it became too difficult to reproduce my testing setup on mainline. This patch still seems correct and relevant to upstream, so I'm posting it. If this is not acceptible to you then please ignore this patch. Also note that when I tested the Chrome OS 3.14 kernel tree I couldn't reproduce the long delays described in the patch. Presumably something changed in either the kernel config or the memory management code between the two kernel versions that made this crop up. In a similar vein, it is possible that problems described in this patch are no longer reproducible upstream. However, the arguments made in this patch (that we don't want to block while holding the mutex) still apply so I think the patch may still have merit. drivers/md/dm-bufio.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)