linux-kernel - Re: [PATCH 2/3] sched, timer: Use atomics for thread_group

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Date:	Wed, 15 Apr 2015 16:07:03 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>, hideaki.kimura@...com,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability

On 04/15/2015 04:39 AM, Jason Low wrote:
>  /*
> @@ -885,11 +890,8 @@ static void check_thread_timers(struct task_struct *tsk,
>  static void stop_process_timers(struct signal_struct *sig)
>  {
>  	struct thread_group_cputimer *cputimer = &sig->cputimer;
> -	unsigned long flags;
> 
> -	raw_spin_lock_irqsave(&cputimer->lock, flags);
> -	cputimer->running = 0;
> -	raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> +	WRITE_ONCE(cputimer->running, 0);

Why do a WRITE_ONCE() here ? Maybe you should explicitly mention this
through a comment like Steven pointed out about all
WRITE/READ/ACCESS_ONCE() usage.

Regards
Preeti U Murthy
>  }
> 
>  static u32 onecputick;
> @@ -1114,9 +1116,7 @@ static inline int fastpath_timer_check(struct task_struct *tsk)
>  	if (sig->cputimer.running) {
>  		struct task_cputime group_sample;
> 
> -		raw_spin_lock(&sig->cputimer.lock);
> -		group_sample = sig->cputimer.cputime;
> -		raw_spin_unlock(&sig->cputimer.lock);
> +		sample_group_cputimer(&group_sample, &sig->cputimer);
> 
>  		if (task_cputime_expired(&group_sample, &sig->cputime_expires))
>  			return 1;
> 

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