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Revert "mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()"

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This reverts commit 1643db98d9.

Kernel test robot reports -88.8% regression in stress-ng.ramfs.ops_per_sec
test case [1], which is caused by commit f95bdb700b ("mm: vmscan: make
global slab shrink lockless").  The root cause is that SRCU has to be
careful to not frequently check for SRCU read-side critical section exits.
Therefore, even if no one is currently in the SRCU read-side critical
section, synchronize_srcu() cannot return quickly.  That's why
unregister_shrinker() has become slower.

We will try to use the refcount+RCU method [2] proposed by Dave Chinner to
continue to re-implement the lockless slab shrink.  So we still need
shrinker_rwsem in synchronize_shrinkers() after reverting the
shrinker_srcu related changes.

[1]. https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@intel.com/
[2]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@dread.disaster.area/

Link: https://lkml.kernel.org/r/20230609081518.3039120-3-qi.zheng@linux.dev
Reported-by: kernel test robot <yujie.liu@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@intel.com
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Kirill Tkhai <tkhai@ya.ru>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Qi Zheng 2023-06-09 08:15:13 +00:00 committed by Andrew Morton
parent 47a7c01c3e
commit 07252b0f97
1 changed files with 6 additions and 2 deletions
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8
mm/vmscan.c
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@ -831,11 +831,15 @@ EXPORT_SYMBOL(unregister_shrinker);
/**
* synchronize_shrinkers - Wait for all running shrinkers to complete.
*
* This is useful to guarantee that all shrinker invocations have seen an
* update, before freeing memory.
* This is equivalent to calling unregister_shrink() and register_shrinker(),
* but atomically and with less overhead. This is useful to guarantee that all
* shrinker invocations have seen an update, before freeing memory, similar to
* rcu.
*/
void synchronize_shrinkers(void)
{
down_write(&shrinker_rwsem);
up_write(&shrinker_rwsem);
atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
}