linux-kernel - Re: [PATCH v2] sched, timer: Use atomics for thread_group

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Date:	Mon, 02 Mar 2015 13:19:32 -0800
From:	Jason Low <jason.low2@...com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Scott Norton <scott.norton@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability

On Mon, 2015-03-02 at 20:40 +0100, Oleg Nesterov wrote:
> Well, I forgot everything about this code, but let me ask anyway ;)
> 
> On 03/02, Jason Low wrote:

> > @@ -222,13 +239,10 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
> >  		 * it.
> >  		 */
> >  		thread_group_cputime(tsk, &sum);
> > -		raw_spin_lock_irqsave(&cputimer->lock, flags);
> > -		cputimer->running = 1;
> > -		update_gt_cputime(&cputimer->cputime, &sum);
> > -	} else
> > -		raw_spin_lock_irqsave(&cputimer->lock, flags);
> > -	*times = cputimer->cputime;
> > -	raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> > +		update_gt_cputime(cputimer, &sum);
> > +		ACCESS_ONCE(cputimer->running) = 1;
> 
> WRITE_ONCE() looks better... 

Okay, I can update that.

> but it is not clear to me why do we need it
> at all.

Peter suggested it here as we would now be updating the running field
without the lock:

https://lkml.org/lkml/2015/1/23/641

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