[PATCH 1/2] mm/vmscan: add sync_shrinkers function v2

[Daniel Vetter]

On Fri, Aug 20, 2021 at 02:05:27PM +0200, Christian König wrote:
> From: Christian König <ckoenig.leichtzumerken at gmail.com>
> 
> While unplugging a device the TTM shrinker implementation
> needs a barrier to make sure that all concurrent shrink
> operations are done and no other CPU is referring to a
> device specific pool any more.
> 
> Taking and releasing the shrinker semaphore on the write
> side after unmapping and freeing all pages from the device
> pool should make sure that no shrinker is running in
> paralell.
> 
> This allows us to avoid the contented mutex in the TTM pool
> implementation for every alloc/free operation.
> 
> v2: rework the commit message to make clear why we need this
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> Acked-by: Huang Rui <ray.huang at amd.com>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> ---
>  include/linux/shrinker.h |  1 +
>  mm/vmscan.c              | 10 ++++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 9814fff58a69..1de17f53cdbc 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -93,4 +93,5 @@ extern void register_shrinker_prepared(struct shrinker *shrinker);
>  extern int register_shrinker(struct shrinker *shrinker);
>  extern void unregister_shrinker(struct shrinker *shrinker);
>  extern void free_prealloced_shrinker(struct shrinker *shrinker);
> +extern void sync_shrinkers(void);
>  #endif
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4620df62f0ff..fde1aabcfa7f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -638,6 +638,16 @@ void unregister_shrinker(struct shrinker *shrinker)
>  }
>  EXPORT_SYMBOL(unregister_shrinker);
>  
> +/**
> + * sync_shrinker - Wait for all running shrinkers to complete.

I think it would be good to add a bit more text here maybe:

"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."

Also a bit a bikeshed, but if we look at the equivalent in irq land it's
called synchronize_irq() and synchronize_hardirq(). I think it'd be good
to bikeshed that for more conceptual consistency.
-Daniel

> + */
> +void sync_shrinkers(void)
> +{
> +	down_write(&shrinker_rwsem);
> +	up_write(&shrinker_rwsem);
> +}
> +EXPORT_SYMBOL(sync_shrinkers);
> +
>  #define SHRINK_BATCH 128
>  
>  static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch