linux-kernel - Re: [PATCH 2/2] memcg: coalescing charge by percpu (Oct/9)

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Date:	Fri, 9 Oct 2009 16:50:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	h-shimamoto@...jp.nec.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] memcg: coalescing charge by percpu (Oct/9)

On Fri, 9 Oct 2009 17:01:05 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> +static void drain_all_stock_async(void)
> +{
> +	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_pending() will catch if there is a race. We just do
> +	 * loose check here.
> +	 */
> +	if (atomic_read(&memcg_drain_count))
> +		return;
> +	/* Notify other cpus that system-wide "drain" is running */
> +	atomic_inc(&memcg_drain_count);
> +	get_online_cpus();
> +	for_each_online_cpu(cpu) {
> +		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> +		if (work_pending(&stock->work))
> +			continue;
> +		INIT_WORK(&stock->work, drain_local_stock);
> +		schedule_work_on(cpu, &stock->work);
> +	}
> + 	put_online_cpus();
> +	atomic_dec(&memcg_drain_count);
> +	/* We don't wait for flush_work */
> +}

It's unusual to run INIT_WORK() each time we use a work_struct. 
Usually we will run INIT_WORK a single time, then just repeatedly use
that structure.  Because after the work has completed, it is still in a
ready-to-use state.

Running INIT_WORK() repeatedly against the same work_struct adds a risk
that we'll scribble on an in-use work_struct, which would make a big
mess.

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