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Date: Mon, 2 Mar 2015 20:43:56 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Jason Low <jason.low2@hp.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
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
	to improve scalability
Message-ID: <20150302194356.GB27914@redhat.com>
References: <1425321731.5304.14.camel@j-VirtualBox> <20150302194033.GA27914@redhat.com>
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On 03/02, Oleg Nesterov wrote:
>
> Well, I forgot everything about this code, but let me ask anyway ;)
>
> On 03/02, Jason Low wrote:
> >
> > -static void update_gt_cputime(struct task_cputime *a, struct task_cputime *b)
> > +static inline void __update_gt_cputime(atomic64_t *cputime, u64 sum_cputime)
> >  {
> > -	if (b->utime > a->utime)
> > -		a->utime = b->utime;
> > -
> > -	if (b->stime > a->stime)
> > -		a->stime = b->stime;
> > +	u64 curr_cputime;
> > +	/*
> > +	 * Set cputime to sum_cputime if sum_cputime > cputime. Use cmpxchg
> > +	 * to avoid race conditions with concurrent updates to cputime.
> > +	 */
> > +retry:
> > +	curr_cputime = atomic64_read(cputime);
> > +	if (sum_cputime > curr_cputime) {
> > +		if (atomic64_cmpxchg(cputime, curr_cputime, sum_cputime) != curr_cputime)
> > +			goto retry;
> > +	}
> > +}
> >
> > -	if (b->sum_exec_runtime > a->sum_exec_runtime)
> > -		a->sum_exec_runtime = b->sum_exec_runtime;
> > +static void update_gt_cputime(struct thread_group_cputimer *cputimer, struct task_cputime *sum)
> > +{
> > +	__update_gt_cputime(&cputimer->utime, sum->utime);
> > +	__update_gt_cputime(&cputimer->stime, sum->stime);
> > +	__update_gt_cputime(&cputimer->sum_exec_runtime, sum->sum_exec_runtime);
> >  }
>
> And this is called if !cputimer_running().
>
> So who else can update these atomic64_t's ? The caller is called under ->siglock.
> IOW, do we really need to cmpxchg/retry ?
>
> Just curious, I am sure I missed something.

Ah, sorry, I seem to understand.

We still can race with account_group_*time() even if ->running == 0. Because
(say) account_group_exec_runtime() can race with 1 -> 0 -> 1 transition.

Or is there another reason?

Oleg.