[Devel] [PATCH RHEL7 COMMIT] ms/trylock_super(): replacement for grab_super_passive()
Konstantin Khorenko
khorenko at virtuozzo.com
Fri Oct 13 15:40:02 MSK 2017
The commit is pushed to "branch-rh7-3.10.0-693.1.1.vz7.37.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.1.1.vz7.37.13
------>
commit 22c1ffe395aa3d560ba33139aeb38a8840783766
Author: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Date: Fri Oct 13 15:40:02 2017 +0300
ms/trylock_super(): replacement for grab_super_passive()
We have a softlockup in wb_do_writeback -> wb_writeback ->
__writeback_inodes_wb stack, as we iterate over works in
wb->bdi->work_list and for each work we move wb->b_dirty inodes to
wb->b_io(in queue_io), and later in __writeback_inodes_wb() we try
to write them, we trylock inode->i_sb->s_umount rw_semaphore and
fail, we put each inode back to wb->b_dirty, and on each inode we
take sb_lock - which can be long. We do aproximately ~100.000.000
iterations, witch takes more than softlockup threshold(20sec).
The above s_umount lock is taken by sync() in container in
sync_filesystems_ve() wich sleeps for 50 sec.
Removing sb_lock from the equation should remove most of contention
and all iterations should pass much faster.
https://jira.sw.ru/browse/PSBM-73370
ms commit: eb6ef3d ("trylock_super(): replacement for grab_super_passive()")
original commit message:
I've noticed significant locking contention in memory reclaimer around
sb_lock inside grab_super_passive(). Grab_super_passive() is called from
two places: in icache/dcache shrinkers (function super_cache_scan) and
from writeback (function __writeback_inodes_wb). Both are required for
progress in memory allocator.
Grab_super_passive() acquires sb_lock to increment sb->s_count and check
sb->s_instances. It seems sb->s_umount locked for read is enough here:
super-block deactivation always runs under sb->s_umount locked for write.
Protecting super-block itself isn't a problem: in super_cache_scan() sb
is protected by shrinker_rwsem: it cannot be freed if its slab shrinkers
are still active. Inside writeback super-block comes from inode from bdi
writeback list under wb->list_lock.
This patch removes locking sb_lock and checks s_instances under s_umount:
generic_shutdown_super() unlinks it under sb->s_umount locked for write.
New variant is called trylock_super() and since it only locks semaphore,
callers must call up_read(&sb->s_umount) instead of drop_super(sb) when
they're done.
Signed-off-by: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
Suggested-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
fs/fs-writeback.c | 6 +++---
fs/internal.h | 2 +-
fs/super.c | 42 +++++++++++++++++++-----------------------
3 files changed, 23 insertions(+), 27 deletions(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 91f1d0a..7cea021 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -797,9 +797,9 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
if (time_is_before_jiffies(start_time + 15* HZ))
trace = 1;
- if (!grab_super_passive(sb)) {
+ if (!trylock_super(sb)) {
/*
- * grab_super_passive() may fail consistently due to
+ * trylock_super() may fail consistently due to
* s_umount being grabbed by someone else. Don't use
* requeue_io() to avoid busy retrying the inode/sb.
*/
@@ -810,7 +810,7 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
continue;
}
wrote += writeback_sb_inodes(sb, wb, work);
- drop_super(sb);
+ up_read(&sb->s_umount);
/* refer to the same tests at the end of writeback_sb_inodes */
if (wrote) {
diff --git a/fs/internal.h b/fs/internal.h
index 289437c..ab2f706 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -86,7 +86,7 @@ extern struct file *get_empty_filp(void);
* super.c
*/
extern int do_remount_sb(struct super_block *, int, void *, int);
-extern bool grab_super_passive(struct super_block *sb);
+extern bool trylock_super(struct super_block *sb);
extern struct dentry *mount_fs(struct file_system_type *,
int, const char *, void *);
extern struct super_block *user_get_super(dev_t);
diff --git a/fs/super.c b/fs/super.c
index af7ae96..3b59975 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -94,8 +94,8 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
if (!(sc->gfp_mask & __GFP_FS))
return SHRINK_STOP;
- if (!grab_super_passive(sb))
- return SHRINK_STOP;
+ if (!trylock_super(sb))
+ return SHRINK_STOP;
if (sb->s_op && sb->s_op->nr_cached_objects)
fs_objects = sb->s_op->nr_cached_objects(sb, sc);
@@ -123,7 +123,7 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
freed += sb->s_op->free_cached_objects(sb, sc);
}
- drop_super(sb);
+ up_read(&sb->s_umount);
return freed;
}
@@ -139,7 +139,7 @@ static unsigned long super_cache_count(struct shrinker *shrink,
sb = container_of(shrink, struct super_block, s_shrink);
/*
- * Don't call grab_super_passive as it is a potential
+ * Don't call trylock_super as it is a potential
* scalability bottleneck. The counts could get updated
* between super_cache_count and super_cache_scan anyway.
* Call to super_cache_count with shrinker_rwsem held
@@ -373,35 +373,31 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
}
/*
- * grab_super_passive - acquire a passive reference
+ * trylock_super - try to grab ->s_umount shared
* @sb: reference we are trying to grab
*
- * Tries to acquire a passive reference. This is used in places where we
+ * Try to prevent fs shutdown. This is used in places where we
* cannot take an active reference but we need to ensure that the
- * superblock does not go away while we are working on it. It returns
- * false if a reference was not gained, and returns true with the s_umount
- * lock held in read mode if a reference is gained. On successful return,
- * the caller must drop the s_umount lock and the passive reference when
- * done.
+ * filesystem is not shut down while we are working on it. It returns
+ * false if we cannot acquire s_umount or if we lose the race and
+ * filesystem already got into shutdown, and returns true with the s_umount
+ * lock held in read mode in case of success. On successful return,
+ * the caller must drop the s_umount lock when done.
+ *
+ * Note that unlike get_super() et.al. this one does *not* bump ->s_count.
+ * The reason why it's safe is that we are OK with doing trylock instead
+ * of down_read(). There's a couple of places that are OK with that, but
+ * it's very much not a general-purpose interface.
*/
-bool grab_super_passive(struct super_block *sb)
+bool trylock_super(struct super_block *sb)
{
- spin_lock(&sb_lock);
- if (hlist_unhashed(&sb->s_instances)) {
- spin_unlock(&sb_lock);
- return false;
- }
-
- sb->s_count++;
- spin_unlock(&sb_lock);
-
if (down_read_trylock(&sb->s_umount)) {
- if (sb->s_root && (sb->s_flags & MS_BORN))
+ if (!hlist_unhashed(&sb->s_instances) &&
+ sb->s_root && (sb->s_flags & MS_BORN))
return true;
up_read(&sb->s_umount);
}
- put_super(sb);
return false;
}
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