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 17:20:54 -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 15:09 -0400, KOSAKI Motohiro wrote:
> (4/29/13 2:54 PM), Olivier Langlois wrote:
> > 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()
> 
> Only account when cputimer->running. Quick Quiz: When turn on cputimer->running?
> 
> 
> > 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.
> 
> tick lost doesn't occur an issue. because glibc only test posix conformance and
> posix allow inacculacy. In other words, timer must not run faster than real clock.
> but lost and makes slower are accepted in the test.
> 
What is lost isn't cputimer tick. They are accounted
account_group_exec_runtime(). What is lost it is what is added to
curr->sum_exec_runtime. Thus making the thread group clock running
slower than the cputimer.

Please spend some time reading the code and less time writing e-mails.

Read the code of release_task() and where it is called in do_exit().

Once it is done, it should be clear to you.


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