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    SubjectRe: [PATCH resend] Reading POSIX CPU timer from outside the process.
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    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@jabber.org
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