Re: [PATCH 3/4] drm/ttm: add debugfs entry to test pool shrinker

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Dec 18, 2020 at 06:55:37PM +0100, Christian König wrote:
> Useful for testing.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/gpu/drm/ttm/ttm_pool.c | 50 ++++++++++++++++++++++------------
>  1 file changed, 32 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
> index 1d61e8fc0e81..1cdacd58753a 100644
> --- a/drivers/gpu/drm/ttm/ttm_pool.c
> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> @@ -529,6 +529,28 @@ void ttm_pool_fini(struct ttm_pool *pool)
>  }
>  EXPORT_SYMBOL(ttm_pool_fini);
>  
> +/* As long as pages are available make sure to release at least one */
> +static unsigned long ttm_pool_shrinker_scan(struct shrinker *shrink,
> +					    struct shrink_control *sc)
> +{
> +	unsigned long num_freed = 0;
> +
> +	do
> +		num_freed += ttm_pool_shrink();
> +	while (!num_freed && atomic_long_read(&allocated_pages));
> +
> +	return num_freed;
> +}
> +
> +/* Return the number of pages available or SHRINK_EMPTY if we have none */
> +static unsigned long ttm_pool_shrinker_count(struct shrinker *shrink,
> +					     struct shrink_control *sc)
> +{
> +	unsigned long num_pages = atomic_long_read(&allocated_pages);
> +
> +	return num_pages ? num_pages : SHRINK_EMPTY;
> +}
> +
>  #ifdef CONFIG_DEBUG_FS
>  /* Count the number of pages available in a pool_type */
>  static unsigned int ttm_pool_type_count(struct ttm_pool_type *pt)
> @@ -633,29 +655,19 @@ int ttm_pool_debugfs(struct ttm_pool *pool, struct seq_file *m)
>  }
>  EXPORT_SYMBOL(ttm_pool_debugfs);
>  
> -#endif
> -
> -/* As long as pages are available make sure to release at least one */
> -static unsigned long ttm_pool_shrinker_scan(struct shrinker *shrink,
> -					    struct shrink_control *sc)
> +/* Test the shrinker functions and dump the result */
> +static int ttm_pool_debugfs_shrink_show(struct seq_file *m, void *data)
>  {
> -	unsigned long num_freed = 0;
> +	struct shrink_control sc = { .gfp_mask = GFP_KERNEL };
>  
> -	do
> -		num_freed += ttm_pool_shrink();
> -	while (!num_freed && atomic_long_read(&allocated_pages));

Need to wrap this in fs_reclaim_acquire/release(), otherwise lockdep won't
connect the dots and the dedicated testing interface here isn't very
useful.

Also this is a bit a confusing interface, I'd expect the show function to
do the counting, and then the write function would try to write shrink as
many objects as requests with the _scan function.

Both should be annotated with fs_reclaim_acquire/release.

> +	seq_printf(m, "%lu/%lu\n", ttm_pool_shrinker_count(&mm_shrinker, &sc),
> +		   ttm_pool_shrinker_scan(&mm_shrinker, &sc));
>  
> -	return num_freed;
> +	return 0;
>  }
> +DEFINE_SHOW_ATTRIBUTE(ttm_pool_debugfs_shrink);
>  
> -/* Return the number of pages available or SHRINK_EMPTY if we have none */
> -static unsigned long ttm_pool_shrinker_count(struct shrinker *shrink,
> -					     struct shrink_control *sc)
> -{
> -	unsigned long num_pages = atomic_long_read(&allocated_pages);
> -
> -	return num_pages ? num_pages : SHRINK_EMPTY;
> -}
> +#endif
>  
>  /**
>   * ttm_pool_mgr_init - Initialize globals
> @@ -688,6 +700,8 @@ int ttm_pool_mgr_init(unsigned long num_pages)
>  #ifdef CONFIG_DEBUG_FS
>  	debugfs_create_file("page_pool", 0444, ttm_debugfs_root, NULL,
>  			    &ttm_pool_debugfs_globals_fops);
> +	debugfs_create_file("page_pool_shrink", 0400, ttm_debugfs_root, NULL,
> +			    &ttm_pool_debugfs_shrink_fops);

I think an interface per pool would make more sense for dedicated testing.
I'm kinda wondering whether we shouldn't do this in mm/ for all shrinkers
by default, it's really quite neat for being able to test stuff.
-Daniel

>  #endif
>  
>  	mm_shrinker.count_objects = ttm_pool_shrinker_count;
> -- 
> 2.25.1
> 

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