diff options
author | Douglas Anderson <dianders@chromium.org> | 2016-11-17 11:24:20 -0800 |
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committer | Mike Snitzer <snitzer@redhat.com> | 2016-12-08 14:13:04 -0500 |
commit | 9ea61cac0b1ad0c09022f39fd97e9b99a2cfc2dc (patch) | |
tree | 9bfef6421952842d4e2ffb3fdf3316b1d0583322 /drivers/md/dm-bufio.c | |
parent | 5b8c01f74cf03b1ec013fcf358b384706233f2f0 (diff) | |
download | linux-9ea61cac0b1ad0c09022f39fd97e9b99a2cfc2dc.tar.bz2 |
dm bufio: avoid sleeping while holding the dm_bufio lock
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).
This change significantly increases responsiveness of the system while
in this state. It makes a real difference because it unblocks kswapd.
In the bug report analyzed, kswapd was hung:
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
By unblocking kswapd memory pressure should be reduced.
Suggested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'drivers/md/dm-bufio.c')
-rw-r--r-- | drivers/md/dm-bufio.c | 5 |
1 files changed, 3 insertions, 2 deletions
diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c index 125aedc3875f..b5d3428d109e 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 +838,7 @@ 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; } |