linux-kernel - Re: [PATCH 1/4] memcg: do not try to drain per-cpu caches without pages

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Date:	Thu, 21 Jul 2011 19:12:50 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] memcg: do not try to drain per-cpu caches without
 pages

On Thu, 21 Jul 2011 09:38:00 +0200
Michal Hocko <mhocko@...e.cz> wrote:

> drain_all_stock_async tries to optimize a work to be done on the work
> queue by excluding any work for the current CPU because it assumes that
> the context we are called from already tried to charge from that cache
> and it's failed so it must be empty already.
> While the assumption is correct we can do it by checking the current
> number of pages in the cache. This will also reduce a work on other CPUs
> with an empty stock.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>


At the first look, when a charge against TransParentHugepage() goes
into the reclaim routine, stock->nr_pages != 0 and this will
call additional kworker.

nr_pages check itself seems good.

Thanks,
-Kame

> ---
>  mm/memcontrol.c |   14 ++------------
>  1 files changed, 2 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f11f198..786bffb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2140,7 +2140,7 @@ static void refill_stock(struct mem_cgroup *mem, unsigned int nr_pages)
>   */
>  static void drain_all_stock_async(struct mem_cgroup *root_mem)
>  {
> -	int cpu, curcpu;
> +	int cpu;
>  	/*
>  	 * If someone calls draining, avoid adding more kworker runs.
>  	 */
> @@ -2148,22 +2148,12 @@ static void drain_all_stock_async(struct mem_cgroup *root_mem)
>  		return;
>  	/* Notify other cpus that system-wide "drain" is running */
>  	get_online_cpus();
> -	/*
> -	 * Get a hint for avoiding draining charges on the current cpu,
> -	 * which must be exhausted by our charging.  It is not required that
> -	 * this be a precise check, so we use raw_smp_processor_id() instead of
> -	 * getcpu()/putcpu().
> -	 */
> -	curcpu = raw_smp_processor_id();
>  	for_each_online_cpu(cpu) {
>  		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
>  		struct mem_cgroup *mem;
>  
> -		if (cpu == curcpu)
> -			continue;
> -
>  		mem = stock->cached;
> -		if (!mem)
> +		if (!mem || !stock->nr_pages)
>  			continue;
>  		if (mem != root_mem) {
>  			if (!root_mem->use_hierarchy)
> -- 
> 1.7.5.4
> 
> 
> 

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