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

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Message-ID: <20141219170334.GM30905@twins.programming.kicks-ass.net>
Date:	Fri, 19 Dec 2014 18:03:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Shaohua Li <shli@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Kernel-team@...com,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO

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@...radead.org> 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.
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