Re: [PATCH v6 7/7] zsmalloc: use shrinker to trigger auto-compaction

[Minchan Kim <minchan@kernel.org>]

On Tue, Jul 07, 2015 at 08:57:01PM +0900, Sergey Senozhatsky wrote:
> Perform automatic pool compaction by a shrinker when system
> is getting tight on memory.
> 
> User-space has a very little knowledge regarding zsmalloc fragmentation
> and basically has no mechanism to tell whether compaction will result
> in any memory gain. Another issue is that user space is not always
> aware of the fact that system is getting tight on memory. Which leads
> to very uncomfortable scenarios when user space may start issuing
> compaction 'randomly' or from crontab (for example). Fragmentation
> is not always necessarily bad, allocated and unused objects, after all,
> may be filled with the data later, w/o the need of allocating a new
> zspage. On the other hand, we obviously don't want to waste memory
> when the system needs it.
> 
> Compaction now has a relatively quick pool scan so we are able to
> estimate the number of pages that will be freed easily, which makes it
> possible to call this function from a shrinker->count_objects() callback.
> We also abort compaction as soon as we detect that we can't free any
> pages any more, preventing wasteful objects migrations.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Suggested-by: Minchan Kim <minchan@kernel.org>


There is one suggestion. Please see below.

> ---
>  mm/zsmalloc.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 74 insertions(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 13f2c4a..83b2e97 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -247,6 +247,10 @@ struct zs_pool {
>  	atomic_long_t		pages_allocated;
>  
>  	struct zs_pool_stats	stats;
> +
> +	/* Compact classes */
> +	struct shrinker		shrinker;
> +	bool			shrinker_enabled;
>  #ifdef CONFIG_ZSMALLOC_STAT
>  	struct dentry		*stat_dentry;
>  #endif
> @@ -1787,6 +1791,69 @@ void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats)
>  }
>  EXPORT_SYMBOL_GPL(zs_pool_stats);
>  
> +static unsigned long zs_shrinker_scan(struct shrinker *shrinker,
> +		struct shrink_control *sc)
> +{
> +	unsigned long pages_freed;
> +	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
> +			shrinker);
> +
> +	pages_freed = pool->stats.pages_compacted;
> +	/*
> +	 * Compact classes and calculate compaction delta.
> +	 * Can run concurrently with a manually triggered
> +	 * (by user) compaction.
> +	 */
> +	pages_freed = zs_compact(pool) - pages_freed;
> +
> +	return pages_freed ? pages_freed : SHRINK_STOP;
> +}
> +
> +static unsigned long zs_shrinker_count(struct shrinker *shrinker,
> +		struct shrink_control *sc)
> +{
> +	int i;
> +	struct size_class *class;
> +	unsigned long pages_to_free = 0;
> +	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
> +			shrinker);
> +
> +	if (!pool->shrinker_enabled)
> +		return 0;
> +
> +	for (i = zs_size_classes - 1; i >= 0; i--) {
> +		class = pool->size_class[i];
> +		if (!class)
> +			continue;
> +		if (class->index != i)
> +			continue;
> +
> +		spin_lock(&class->lock);
> +		pages_to_free += zs_can_compact(class);
> +		spin_unlock(&class->lock);
> +	}
> +
> +	return pages_to_free;
> +}
> +
> +static void zs_unregister_shrinker(struct zs_pool *pool)
> +{
> +	if (pool->shrinker_enabled) {
> +		unregister_shrinker(&pool->shrinker);
> +		pool->shrinker_enabled = false;
> +	}
> +}
> +
> +static int zs_register_shrinker(struct zs_pool *pool)
> +{
> +	pool->shrinker.scan_objects = zs_shrinker_scan;
> +	pool->shrinker.count_objects = zs_shrinker_count;
> +	pool->shrinker.batch = 0;
> +	pool->shrinker.seeks = DEFAULT_SEEKS;
> +
> +	return register_shrinker(&pool->shrinker);
> +}
> +
>  /**
>   * zs_create_pool - Creates an allocation pool to work from.
>   * @flags: allocation flags used to allocate pool metadata
> @@ -1872,6 +1939,12 @@ struct zs_pool *zs_create_pool(char *name, gfp_t flags)
>  	if (zs_pool_stat_create(name, pool))
>  		goto err;
>  
> +	/*
> +	 * Not critical, we still can use the pool
> +	 * and user can trigger compaction manually.
> +	 */
> +	if (zs_register_shrinker(pool) == 0)
> +		pool->shrinker_enabled = true;

IMO, there is no value to maintain just in case of
failing register_shrinker in practice.

Let's remove shrinker_enabled and abort pool creation if shrinker register
is failed.

Tomorrow, I will test this patchset and add Acked-by if it pass.

Thanks!


>  	return pool;
>  
>  err:
> @@ -1884,6 +1957,7 @@ void zs_destroy_pool(struct zs_pool *pool)
>  {
>  	int i;
>  
> +	zs_unregister_shrinker(pool);
>  	zs_pool_stat_destroy(pool);
>  
>  	for (i = 0; i < zs_size_classes; i++) {
> -- 
> 2.4.5
> 

-- 
Kind regards,
Minchan Kim

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