linux-kernel - Re: [RFC PATCH] timer: Improve itimers scalability

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Date:	Thu, 6 Aug 2015 16:18:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Mike Galbraith <umgwanakikbuti@...il.com>, terry.rudd@...com,
	Rik van Riel <riel@...hat.com>,
	Waiman Long <Waiman.Long@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [RFC PATCH] timer: Improve itimers scalability

On 08/04, Jason Low wrote:
>
> @@ -973,13 +981,6 @@ static void check_process_timers(struct task_struct *tsk,
>  	virt_expires = check_timers_list(++timers, firing, utime);
>  	sched_expires = check_timers_list(++timers, firing, sum_sched_runtime);
>  
> -	/*
> -	 * Check for the special case process timers.
> -	 */
> -	check_cpu_itimer(tsk, &sig->it[CPUCLOCK_PROF], &prof_expires, ptime,
> -			 SIGPROF);
> -	check_cpu_itimer(tsk, &sig->it[CPUCLOCK_VIRT], &virt_expires, utime,
> -			 SIGVTALRM);
>  	soft = READ_ONCE(sig->rlim[RLIMIT_CPU].rlim_cur);
>  	if (soft != RLIM_INFINITY) {
>  		unsigned long psecs = cputime_to_secs(ptime);
> @@ -1010,11 +1011,21 @@ static void check_process_timers(struct task_struct *tsk,
>  		}
>  	}
>  
> +	/*
> +	 * Check for the special case process timers.
> +	 */
> +	check_cpu_itimer(tsk, &sig->it[CPUCLOCK_PROF], &prof_expires, ptime,
> +			 SIGPROF);
> +	check_cpu_itimer(tsk, &sig->it[CPUCLOCK_VIRT], &virt_expires, utime,
> +			 SIGVTALRM);
> +

Not sure I understand this part... looks wrong actually, please note
that RLIMIT_CPU block above may need to update prof_expires _after_
check_cpu_itimer(), or I am totally confused.

>  	if (READ_ONCE(sig->cputimer.running)) {
>  		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;
> +

Cosmetic, I won't insist, but this is not symmetrical to ->running check,

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

looks a littke bit better to me.

Oleg.

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