Messages in this thread | | | Date | Thu, 9 Mar 2023 15:06:25 +0800 | Subject | Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers() | From | Qi Zheng <> |
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Hi Kirill,
On 2023/3/9 06:39, Kirill Tkhai wrote: > On 07.03.2023 09:56, Qi Zheng wrote: >> Now there are no readers of shrinker_rwsem, so >> synchronize_shrinkers() does not need to hold the >> writer of shrinker_rwsem to wait for all running >> shinkers to complete, synchronize_srcu() is enough. >> >> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> >> --- >> mm/vmscan.c | 8 ++------ >> 1 file changed, 2 insertions(+), 6 deletions(-) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 7aaf6f94ac1b..ac7ab4aa344f 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -796,15 +796,11 @@ EXPORT_SYMBOL(unregister_shrinker); >> /** >> * synchronize_shrinkers - Wait for all running shrinkers to complete. >> * >> - * 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. >> + * This is useful to guarantee that all shrinker invocations have seen an >> + * update, before freeing memory. >> */ >> void synchronize_shrinkers(void) >> { >> - down_write(&shrinker_rwsem); >> - up_write(&shrinker_rwsem); >> atomic_inc(&shrinker_srcu_generation); >> synchronize_srcu(&shrinker_srcu); >> } > > Just curious, callers of synchronize_shrinkers() don't want to have parallel register_shrinker() and unregister_shrink() are completed? > Here we only should wait for parallel shrink_slab(), correct?
I think yes.
The synchronize_shrinkers() is currently only used by TTM pool.
In TTM pool, a shrinker named "drm-ttm_pool" is registered, and the scan_objects callback will pick an entry from its own shrinker_list:
ttm_pool_shrink --> spin_lock(&shrinker_lock); pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list); list_move_tail(&pt->shrinker_list, &shrinker_list); spin_unlock(&shrinker_lock);
These entries have been removed from shrinker_list before calling synchronize_shrinkers():
ttm_pool_fini --> ttm_pool_type_fini --> spin_lock(&shrinker_lock); list_del(&pt->shrinker_list); spin_unlock(&shrinker_lock); synchronize_shrinkers
So IIUC, we only need to wait for the parallel shrink_slab() here. Like its comment says:
/* We removed the pool types from the LRU, but we need to also make sure * that no shrinker is concurrently freeing pages from the pool. */
+ CC: Christian König :)
Thanks, Qi
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