linux-kernel - Re: [PATCH v2 1/3] process cputimer is moving faster than its corresponding clock

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Date:	Mon, 29 Apr 2013 14:54:27 -0400
From:	Olivier Langlois <olivier@...llion01.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, schwidefsky@...ibm.com,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] process cputimer is moving faster than its
 corresponding clock

On Mon, 2013-04-29 at 14:31 -0400, KOSAKI Motohiro wrote:
> (4/29/13 2:20 PM), Olivier Langlois wrote:
> > 
> >>
> >>>> I'm confused. glibc's rt/tst-cputimer1 doesn't have thread exiting code. I have
> >>>> no seen any issue in this accounting.
> >>>
> >>> glibc launch a helper thread to receive timer signal and will also
> >>> create a new thread upon signal reception when a timer is created with
> >>> sigev_notify = SIGEV_THREAD;
> >>>
> >>> please see:
> >>>
> >>> glibc-2.17/nptl/sysdeps/unix/sysv/linux/timer_create.c
> >>> glibc-2.17/nptl/sysdeps/unix/sysv/linux/timer_routines.c
> >>
> >> I know. I taled thread exiting. not thread creating. And, as far as I can see, only test sig1 can fail,
> >> not thr[12].
> >>
> > Apart from glibc helper thread, the threads created for handling timer
> > firing all do exit immediatly as soon as user callback returns.
> 
> And, libc ensure its exiting finished before starting actual tests. Why such thread exiting
> affect timers code? It shouldn't. becuase signal.cputimer is initialized timer_settime().
> The initialization is incorrect, we should fix initialization.

It doesn't have anything to do with initialisation.

Quick Quiz #1: How does the cputimer tick?
Answer: With calls to account_group_exec_runtime()

Every task updates occuring after release_task() has been called in
do_exit() (scheduler ticks or the task final schedule() call) will be
lost because tasks stats are added to the global group stats located in
the signal struct in release_task() So every update after release_task()
will be lost but account_group_exec_runtime is still called.

Tasks that go in zombie state are fine because release_task() will be
called later. Autoreap task (those with CLONE_THREAD ie: pthreads) calls
release_task() before the last context switch. Please do read
kernel/exit.c.

Hence cputimer advance faster than the process clock.

Hence the POSIX compliance from your pseudo code does not hold

        sighandler(){
                t1 = clock_gettime()
        }

        t0 = clock_gettime()
        timer_settime(timeout);
         ... wait to fire
        
        assert (t1 - t0 >= timeout)

> 
> 
> > I count 12 thread exits during tst-cputimer1 execution. The errors do
> > add up hence you're more likely to see errors after 2.5 sec and up from
> > start of execution. I have seen sig1, thr[12] fails. I see no reason why
> > one could not fail.



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