linux-kernel - Re: [tip:perf/timer] perf: Add per event clockid support

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Message-ID: <CABPqkBT-H8T40FcDZAWahMAajP4NBbMOVs9Awghh598xaoiz9Q@mail.gmail.com>
Date:	Fri, 27 Mar 2015 09:52:25 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	"H. Peter Anvin" <hpa@...or.com>, David Ahern <dsahern@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [tip:perf/timer] perf: Add per event clockid support

On Fri, Mar 27, 2015 at 9:35 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Mar 27, 2015 at 09:31:45AM -0700, Stephane Eranian wrote:
>> On Fri, Mar 27, 2015 at 4:48 AM, tip-bot for Peter Zijlstra
>> > +static int perf_event_set_clock(struct perf_event *event, clockid_t clk_id)
>> > +{
>> > +       bool nmi_safe = false;
>> > +
>> > +       switch (clk_id) {
>> > +       case CLOCK_MONOTONIC:
>> > +               event->clock = &ktime_get_mono_fast_ns;
>> > +               nmi_safe = true;
>> > +               break;
>> > +
>> > +       case CLOCK_MONOTONIC_RAW:
>> > +               event->clock = &ktime_get_raw_fast_ns;
>> > +               nmi_safe = true;
>> > +               break;
>> > +
>> > +       case CLOCK_REALTIME:
>> > +               event->clock = &ktime_get_real_ns;
>> > +               break;
>> > +
>> > +       case CLOCK_BOOTTIME:
>> > +               event->clock = &ktime_get_boot_ns;
>> > +               break;
>> > +
>> > +       case CLOCK_TAI:
>> > +               event->clock = &ktime_get_tai_ns;
>> > +               break;
>> > +
>> Can all those clocks be safely called from an NMI context?
>
> +       if (!nmi_safe && !(event->pmu->capabilities & PERF_PMU_CAP_NO_NMI))
> +               return -EINVAL;
>
> no :-)

Ok, I see. But on architectures which do not have NMI, they would all
be safe. And
that would work if they set the PERF_PMU_CAP_NO_NMI flag on their
pmu->capabilities.

Next, I am trying to understand how perf is going to expose this. I am
thinking about this
in the context of my jitted code patches. With this approach, the jit
runtime and the perf
tool need to agree on the clock they are using. That mean they need to
advertise or
document the clock they use and there needs to be an option in perf
record to pass
that same clockid.

In my current patch series, using Pawel's clock patch, the kernel advertises in
procfs the clockid used (all events use the same). The jit runtime simply reads
the id and uses it to timestamp jit events.
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