linux-kernel - Re: [PATCH resend] Reading POSIX CPU timer from outside the process.

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Date:	Wed, 29 Dec 2010 15:10:14 +0100
From:	Dario Faggioli <raistlin@...ux.it>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	torbenh <torbenh@....de>, john.stultz@...aro.org,
	roland@...hat.com, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: Re: [PATCH resend] Reading POSIX CPU timer from outside the
 process.

On Wed, 2010-12-29 at 14:21 +0100, Oleg Nesterov wrote: 
> > > How so? For example, with this change
> > > clock_getres(MAKE_THREAD_CPUCLOCK(pid_of_sub_thread)) won't work, no?
> > >
> > I tested all the clock_getres() calls that came to my mind (at least the
> > one that are possible from an userspace program), and they always worked
> > because of this (still in check_clock):
> >
> >         const pid_t pid = CPUCLOCK_PID(which_clock);
> >
> >         if (pid == 0)
> >                 return 0;
> >
> > Which triggers all the times,
> 
> No, please note pid_of_sub_thread above.
> 
Sure, I saw that... I was referring to all the clock_getres() and
clock_{get,set}time() I was able to call without using directly
MAKE_THREAD_CPUCLOCK.

> > Which won't work because CPUCLOCK_PERTHREAD(which_clock) is always false
> > in this case.
> 
> I guess, this is because glibc passes MAKE_PROCESS_CLOCK() id, right?
> But we shouldn't add the hacks to the kernel to hide the limitations
> in glibc.
> 
Exactly. Actually, I didn't noticed this too when I first started with
this patch, and the fact that my first version worked -- which was by
chance, I can say it now :-P -- made me think it was worthwhile to bend
the semantic a bit, to enable this new capability... But making it work
is hackish, I see it now. Sorry. :-)

> BTW. What is the test-case? I am looking at http://gitorious.org/clockid,
> I guess it is clockid.c...
> 
Yep. A very trivial one, I agree, but it's just to show the issue.

> You do not need clock_getcpuclockid() at all. In fact I do not really
> understand what this helper should actually do, probably it is only
> needed to validate the pid. You can simply use MAKE_THREAD_CPUCLOCK()
> to sample a single thread via clock_gettime().
> 
Fine, but, is that macro available for an application developer? Because
I can find it in kernel and glibc sources, but not in my /usr/include/*,
which is the motivation behind this attempt... But it might be my
fault! :-P

> IOW. Unless I missed something, with this patch, the only problem
> is that getcpuclockid() always assumes MAKE_PROCESS_CPUCLOCK(),
> I do not think this is the kernel problem.
> 
Agreed, sorry for wasting (hopefully not too much) people's time. :-(

Thanks and Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://retis.sssup.it/people/faggioli -- dario.faggioli@...ber.org

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