Re: [PATCH] clock_getres.2: Remove obsolete note on SMP systems

[Rodrigo Campos <rodrigo-aOqSs0FX/Gu4Tu3zPC53fQ@public.gmane.org>]

On Wed, Sep 04, 2013 at 02:32:49PM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Rodrigo,
> 
> On Tue, Sep 3, 2013 at 12:05 PM, Rodrigo Campos <rodrigo-aOqSs0FX/Gu4Tu3zPC53fQ@public.gmane.org> wrote:
> > As confirmed by peterz on IRC, this note is obsolete:
> >
> >         <peterz> rata: that section is obsolete; CLOCK_THREAD_CPUTIME_ID is good
> >         <peterz> rata: CLOCK_PROCESS_CPUTIME_ID is also correct
> >
> > So this patch just removes it.
> 
> Clearly, the page needs amending. However, ideal would be to describe
> *when* the section became obsolete (which kernel version). Do you or
> Peter know, or have an idea how we can determine that information?

I don't know. But looking at the git repo, it seems in the first git commit
(1da177e4c3f41524e886b7f1b8a0c1fc7321cac2) it was already safe, because
in kernel/posix-cpu-timers.c cpu_clock_sample() uses the scheduler to get the
value. And the basic pattern:

	posix_cpu_clock_get() --> posix_cpu_clock_get() --> cpu_clock_sample()

(for the case of CLOCK_THREAD_CPUTIME_ID) seems unchanged. So, if I guess it was
safe on those days too.

And for CLOCK_PROCESS_CPUTIME_ID, something similar happens if I follow the code
correctly. The call chain I see is:

	process_cpu_clock_get() --> posix_cpu_clock_get() -->
	cpu_clock_sample_group() --> cpu_clock_sample_group_locked()

And if I follow the clock macros correctly, ends up calling
cpu_clock_sample_group_locked() with CPUCLOCK_SCHED, which calls the scheduler
and I *expect* that to be safe too.

But I would **really** like if Peter can confirm this, as I really don't know :-)



Btw, if in the first git commit it was safe, should I try to look to the history
before it ?




Thanks a lot,
Rodrigo
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