linux-kernel - Re: utime/stime decreasing on thread exit

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Message-ID: <20091110192327.GA16325@redhat.com>
Date:	Tue, 10 Nov 2009 20:23:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Spencer Candland <spencer@...ehost.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: utime/stime decreasing on thread exit

On 11/10, Stanislaw Gruszka wrote:
>
> On Tue, Nov 10, 2009 at 06:40:08PM +0100, Oleg Nesterov wrote:
> > >
> > > Or stay with task_cputime_expired() but only if cputimer is currently running.
> >
> > Oh. I forgot this code completely, can't comment.
> >
> > Can't we ensure that fastpath_timer_check() never do while_each_thread() ?
>
> Removing possibility to call while_each_tread() from fastpath_timer_check()
> was exactly my intension here, perhaps I was not clear.

Yes, yes, I understand.

I meant, perhaps we can ensure this shouldn't happen "by design", instead
of checking ->running in fastpath_timer_check().

> > IOW, if sig->cputime_expires != 0 then ->running must be true.
> > At least, shouldn't stop_process_timers() clear sig->cputime_expires ?
>
> I'm going to think about that. However as far seems, checking ->running
> explicitly and goto slow patch when is not true is safer solution.

Yes, agreed.


Still. check_process_timers() updates sig->cputime_expires at the end,
but it never clears it. For example,

	if (sched_expires != 0 &&
	    (sig->cputime_expires.sched_exp == 0 ||
	     sig->cputime_expires.sched_exp > sched_expires))
		sig->cputime_expires.sched_exp = sched_expires;

Why?

Now suppose that (say) sig->cputime_expires.sched_exp != 0, there are
no cpu timers, ->running == F.

In this case fastpath_timer_check() always returns T and triggers the
slow path which does nothing, not good.

But yes, I agree, probably we should start with the simple/safe change
as you suggested.

Oleg.

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