Am 09.03.23 um 08:06 schrieb Qi Zheng:
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@xxxxxxxxxxxxx>
---
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.
*/
Yes your analyses is completely correct.
I just didn't wanted to add another SRCU into the critical code paths of
the TTM pool just for device hot plug when I have that functionality
already.
We just make sure that no shrinker is running in parallel with
destruction of the pool. Registering and unregistering is harmless.
Regards,
Christian.
+ CC: Christian König :)
Thanks,
Qi
