Date: Wed, 5 Aug 2015 11:37:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Mike Galbraith <umgwanakikbuti@gmail.com>, terry.rudd@hp.com,
        Rik van Riel <riel@redhat.com>, Waiman Long <Waiman.Long@hp.com>,
        Scott J Norton <scott.norton@hp.com>
Subject: Re: [RFC PATCH] timer: Improve itimers scalability
Message-ID: <20150805093719.GV25159@twins.programming.kicks-ass.net>
References: <1438734584.2927.15.camel@j-VirtualBox>
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On Tue, Aug 04, 2015 at 05:29:44PM -0700, Jason Low wrote:

> @@ -1137,6 +1148,13 @@ static inline int fastpath_timer_check(struct task_struct *tsk)
>  	if (READ_ONCE(sig->cputimer.running)) {

Maybe make that:

	if (READ_ONCE(sig->cputimer.running) &&
	    !READ_ONCE(sig->cputimer.is_checking_timer)) {

>  		struct task_cputime group_sample;
>  
> +		/*
> +		 * If another thread in the group is already checking
> +		 * for the thread group cputimer, then we will skip that.
> +		 */
> +		if (READ_ONCE(sig->cputimer.is_checking_timer))
> +			return 0;
> +
>  		sample_cputime_atomic(&group_sample, &sig->cputimer.cputime_atomic);
>  
>  		if (task_cputime_expired(&group_sample, &sig->cputime_expires))


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