Re: [PATCH v10 01/11] drm/msm/gem: Prevent blocking within shrinker loop

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 08.01.23 um 22:04 schrieb Dmitry Osipenko:
> Consider this scenario:
> 
> 1. APP1 continuously creates lots of small GEMs
> 2. APP2 triggers `drop_caches`
> 3. Shrinker starts to evict APP1 GEMs, while APP1 produces new purgeable
>     GEMs
> 4. msm_gem_shrinker_scan() returns non-zero number of freed pages
>     and causes shrinker to try shrink more
> 5. msm_gem_shrinker_scan() returns non-zero number of freed pages again,
>     goto 4
> 6. The APP2 is blocked in `drop_caches` until APP1 stops producing
>     purgeable GEMs
> 
> To prevent this blocking scenario, check number of remaining pages
> that GPU shrinker couldn't release due to a GEM locking contention
> or shrinking rejection. If there are no remaining pages left to shrink,
> then there is no need to free up more pages and shrinker may break out
> from the loop.
> 
> This problem was found during shrinker/madvise IOCTL testing of
> virtio-gpu driver. The MSM driver is affected in the same way.
> 
> Reviewed-by: Rob Clark <robdclark@gmail.com>
> Fixes: b352ba54a820 ("drm/msm/gem: Convert to using drm_gem_lru")
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>   drivers/gpu/drm/drm_gem.c              | 9 +++++++--
>   drivers/gpu/drm/msm/msm_gem_shrinker.c | 8 ++++++--
>   include/drm/drm_gem.h                  | 4 +++-
>   3 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 59a0bb5ebd85..c6bca5ac6e0f 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1388,10 +1388,13 @@ EXPORT_SYMBOL(drm_gem_lru_move_tail);
>    *
>    * @lru: The LRU to scan
>    * @nr_to_scan: The number of pages to try to reclaim
> + * @remaining: The number of pages left to reclaim
>    * @shrink: Callback to try to shrink/reclaim the object.
>    */
>   unsigned long
> -drm_gem_lru_scan(struct drm_gem_lru *lru, unsigned nr_to_scan,
> +drm_gem_lru_scan(struct drm_gem_lru *lru,
> +		 unsigned int nr_to_scan,
> +		 unsigned long *remaining,
>   		 bool (*shrink)(struct drm_gem_object *obj))
>   {
>   	struct drm_gem_lru still_in_lru;
> @@ -1430,8 +1433,10 @@ drm_gem_lru_scan(struct drm_gem_lru *lru, unsigned nr_to_scan,
>   		 * hit shrinker in response to trying to get backing pages
>   		 * for this obj (ie. while it's lock is already held)
>   		 */
> -		if (!dma_resv_trylock(obj->resv))
> +		if (!dma_resv_trylock(obj->resv)) {
> +			*remaining += obj->size >> PAGE_SHIFT;
>   			goto tail;
> +		}
>   
>   		if (shrink(obj)) {
>   			freed += obj->size >> PAGE_SHIFT;
> diff --git a/drivers/gpu/drm/msm/msm_gem_shrinker.c b/drivers/gpu/drm/msm/msm_gem_shrinker.c
> index 051bdbc093cf..b7c1242014ec 100644
> --- a/drivers/gpu/drm/msm/msm_gem_shrinker.c
> +++ b/drivers/gpu/drm/msm/msm_gem_shrinker.c
> @@ -116,12 +116,14 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
>   	};
>   	long nr = sc->nr_to_scan;
>   	unsigned long freed = 0;
> +	unsigned long remaining = 0;
>   
>   	for (unsigned i = 0; (nr > 0) && (i < ARRAY_SIZE(stages)); i++) {
>   		if (!stages[i].cond)
>   			continue;
>   		stages[i].freed =
> -			drm_gem_lru_scan(stages[i].lru, nr, stages[i].shrink);
> +			drm_gem_lru_scan(stages[i].lru, nr, &remaining,

This function relies in remaining being pre-initialized. That's not 
obvious and error prone. At least, pass-in something like 
&stages[i].remaining that is then initialized internally by 
drm_gem_lru_scan() to zero. And similar to freed, sum up the individual 
stages' remaining here.

TBH I somehow don't like the overall design of how all these functions 
interact with each other. But I also can't really point to the actual 
problem. So it's best to take what you have here; maybe with the change 
I proposed.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

Best regards
Thomas

> +					 stages[i].shrink);
>   		nr -= stages[i].freed;
>   		freed += stages[i].freed;
>   	}
> @@ -132,7 +134,7 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
>   				     stages[3].freed);
>   	}
>   
> -	return (freed > 0) ? freed : SHRINK_STOP;
> +	return (freed > 0 && remaining > 0) ? freed : SHRINK_STOP;
>   }
>   
>   #ifdef CONFIG_DEBUG_FS
> @@ -182,10 +184,12 @@ msm_gem_shrinker_vmap(struct notifier_block *nb, unsigned long event, void *ptr)
>   		NULL,
>   	};
>   	unsigned idx, unmapped = 0;
> +	unsigned long remaining = 0;
>   
>   	for (idx = 0; lrus[idx] && unmapped < vmap_shrink_limit; idx++) {
>   		unmapped += drm_gem_lru_scan(lrus[idx],
>   					     vmap_shrink_limit - unmapped,
> +					     &remaining,
>   					     vmap_shrink);
>   	}
>   
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 772a4adf5287..f1f00fc2dba6 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -476,7 +476,9 @@ int drm_gem_dumb_map_offset(struct drm_file *file, struct drm_device *dev,
>   void drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock);
>   void drm_gem_lru_remove(struct drm_gem_object *obj);
>   void drm_gem_lru_move_tail(struct drm_gem_lru *lru, struct drm_gem_object *obj);
> -unsigned long drm_gem_lru_scan(struct drm_gem_lru *lru, unsigned nr_to_scan,
> +unsigned long drm_gem_lru_scan(struct drm_gem_lru *lru,
> +			       unsigned int nr_to_scan,
> +			       unsigned long *remaining,
>   			       bool (*shrink)(struct drm_gem_object *obj));
>   
>   #endif /* __DRM_GEM_H__ */

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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