[PATCH] dm: Avoid sleeping while holding the dm_bufio lock
From: Mikulas Patocka <mpatocka@redhat.com>
Date: 2016-11-23 20:57:08
Also in:
dm-devel, lkml
Hi The GFP_NOIO allocation frees clean cached pages. The GFP_NOWAIT allocation doesn't. Your patch would incorrectly reuse buffers in a situation when the memory is filled with clean cached pages. Here I'm proposing an alternate patch that first tries GFP_NOWAIT allocation, then drops the lock and tries GFP_NOIO allocation. Note that the root cause why you are seeing this stacktrace is, that your block device is congested - i.e. there are too many requests in the device's queue - and note that fixing this wait won't fix the root cause (congested device). The congestion limits are set in blk_queue_congestion_threshold to 7/8 to 13/16 size of the nr_requests value. If you don't want your device to report the congested status, you can increase /sys/block/<device>/queue/nr_requests - you should test if your chromebook is faster of slower with this setting increased. But note that this setting won't increase the IO-per-second of the device. Mikulas On Thu, 17 Nov 2016, Douglas Anderson wrote:
quoted hunk ↗ jump to hunk
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 -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel
From: Mikulas Patocka <mpatocka@redhat.com> Subject: dm-bufio: drop the lock when doing GFP_NOIO alloaction Drop the lock when doing GFP_NOIO alloaction beacuse the allocation can take some time. Note that we won't do GFP_NOIO allocation when we loop for the second time, because the lock shouldn't be dropped between __wait_for_free_buffer and __get_unclaimed_buffer. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> --- drivers/md/dm-bufio.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) Index: linux-2.6/drivers/md/dm-bufio.c ===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c
+++ linux-2.6/drivers/md/dm-bufio.c@@ -822,11 +822,13 @@ enum new_flag { static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf) { struct dm_buffer *b; + bool tried_noio_alloc = false; /* * 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_NOWAIT: don't sleep and don't release cache * GFP_NOIO: don't recurse into the I/O layer * __GFP_NORETRY: don't retry and rather return failure * __GFP_NOMEMALLOC: don't use emergency reserves
@@ -837,7 +839,7 @@ static struct dm_buffer *__alloc_buffer_ */ 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; }
@@ -845,6 +847,15 @@ static struct dm_buffer *__alloc_buffer_ if (nf == NF_PREFETCH) return NULL; + if (dm_bufio_cache_size_latch != 1 && !tried_noio_alloc) { + dm_bufio_unlock(c); + b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); + dm_bufio_lock(c); + if (b) + return b; + tried_noio_alloc = true; + } + if (!list_empty(&c->reserved_buffers)) { b = list_entry(c->reserved_buffers.next, struct dm_buffer, lru_list);