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

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Date:	Tue, 17 Nov 2009 13:48:52 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Oleg Nesterov <oleg@...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 Tue, Nov 10, 2009 at 08:23:27PM +0100, Oleg Nesterov wrote:
> > > 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().

Rule "sig->cputimer_expire != zero implies sig->cputimer.running == true" is
_almost_ assured (after fix in next mail). IMHO there is only one problem with
that functions: posix_cpu_timer_set() and posix_cpu_timer_schedule().

These functions first call thread_group_cputimer() without
tsk->sighand->siglock (only tasklist_lock is taken) and then
arm_timer(), which setups list and cputime_expires cache.

When there is some timer expiring already we can have situation like below:

cpu_timer_sample_group()
				check_process_timers()
					stop_process_timers()
arm_timer()

At the end we end with cputimer_expire != zero and ->running == false.
Very unlikely situation indeed, but possible. To address this
we can do in arm_timer() something like that:

	if (unlikely(!sig->cputimer.running)) {
		cpu_timer_sample_group()
		bump_cpu_timer();
	}

Since we have this we can do optimization, you proposed here:

http://lkml.org/lkml/2009/3/23/381

Use cputimer->running in fastpath_timer_check(). I'm going to work on
it as well on some other optimizations in posix-cpu-timer.c

> 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.

This is real bug. I will fix it in the next patch. 

Stanislaw
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