linux-kernel - Re: [BUGFIX][PATCH 5/5] memcg: fix percpu cached charge draining frequency

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Date:	Tue, 14 Jun 2011 12:04:12 +0200
From:	Johannes Weiner <jweiner@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"bsingharora@...il.com" <bsingharora@...il.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, Ying Han <yinghan@...gle.com>
Subject: Re: [BUGFIX][PATCH 5/5] memcg: fix percpu cached charge draining
 frequency

On Mon, Jun 13, 2011 at 12:16:48PM +0900, KAMEZAWA Hiroyuki wrote:
> @@ -1670,8 +1670,8 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
>  		victim = mem_cgroup_select_victim(root_mem);
>  		if (victim == root_mem) {
>  			loop++;
> -			if (loop >= 1)
> -				drain_all_stock_async();
> +			if (!check_soft && loop >= 1)
> +				drain_all_stock_async(root_mem);

I agree with Michal, this should be a separate change.

> @@ -2008,26 +2011,50 @@ static void refill_stock(struct mem_cgroup *mem, unsigned int nr_pages)
>   * expects some charges will be back to res_counter later but cannot wait for
>   * it.
>   */
> -static void drain_all_stock_async(void)
> +static void drain_all_stock_async(struct mem_cgroup *root_mem)
>  {
> -	int cpu;
> -	/* This function is for scheduling "drain" in asynchronous way.
> -	 * The result of "drain" is not directly handled by callers. Then,
> -	 * if someone is calling drain, we don't have to call drain more.
> -	 * Anyway, WORK_STRUCT_PENDING check in queue_work_on() will catch if
> -	 * there is a race. We just do loose check here.
> +	int cpu, curcpu;
> +	/*
> +	 * If someone calls draining, avoid adding more kworker runs.
>  	 */
> -	if (atomic_read(&memcg_drain_count))
> +	if (!mutex_trylock(&percpu_charge_mutex))
>  		return;
>  	/* Notify other cpus that system-wide "drain" is running */
> -	atomic_inc(&memcg_drain_count);
>  	get_online_cpus();
> +
> +	/*
> +	 * get a hint for avoiding draining charges on the current cpu,
> +	 * which must be exhausted by our charging. But this is not
> +	 * required to be a precise check, 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);
> -		schedule_work_on(cpu, &stock->work);
> +		struct mem_cgroup *mem;
> +
> +		if (cpu == curcpu)
> +			continue;
> +
> +		mem = stock->cached;
> +		if (!mem)
> +			continue;
> +		if (mem != root_mem) {
> +			if (!root_mem->use_hierarchy)
> +				continue;
> +			/* check whether "mem" is under tree of "root_mem" */
> +			rcu_read_lock();
> +			if (!css_is_ancestor(&mem->css, &root_mem->css)) {
> +				rcu_read_unlock();
> +				continue;
> +			}
> +			rcu_read_unlock();

css_is_ancestor() takes the rcu read lock itself already.
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