Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO

[Andy Lutomirski]

On Fri, Dec 19, 2014 at 9:03 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Dec 19, 2014 at 08:48:07AM -0800, Andy Lutomirski wrote:
>> On Fri, Dec 19, 2014 at 3:23 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > On Thu, Dec 18, 2014 at 04:22:59PM -0800, Andy Lutomirski wrote:
>> >> Bad news: this patch is incorrect, I think.  Take a look at
>> >> update_rq_clock -- it does fancy things involving irq time and
>> >> paravirt steal time.  So this patch could result in extremely
>> >> non-monotonic results.
>> >
>> > Yeah, I'm not sure how (and if) we could make all that work :/
>>
>> I obviously can't comment on what Facebook needs, but if I were
>> rigging something up to profile my own code*, I'd want a count of
>> elapsed time, including user, system, and probably interrupt as well.
>> I would probably not want to count time during which I'm not
>> scheduled, and I would also probably not want to count steal time.
>> The latter makes any implementation kind of nasty.
>>
>> The API presumably doesn't need to be any particular clock id for
>> clock_gettime, and it may not even need to be clock_gettime at all.
>>
>> Is perf self-monitoring good enough for this?  If not, can we make it
>> good enough?
>
> Yeah, I think you should be able to use that. You could count a NOP
> event and simply use its activated time. We have PERF_COUNT_SW_DUMMY for
> such purposes iirc.
>
> The advantage of using perf self profiling is that it (obviously)
> extends to more than just walltime.

Re-asking my old question: would it make sense to add a vdso helper
for the magic self-monitoring interface?  Or, at the very least, we
could try to tidy up the docs a bit.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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