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Message-ID: <1425331172.5304.50.camel@j-VirtualBox>
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability
From: Jason Low <jason.low2@hp.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Rik van Riel <riel@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
        Scott Norton <scott.norton@hp.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>, linux-kernel@vger.kernel.org
Date: Mon, 02 Mar 2015 13:19:32 -0800
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	 <20150302194033.GA27914@redhat.com>
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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