linux-kernel - Re: [PATCH 1/2] perf/x86/intel: enable CPU ref

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Date:   Sun, 28 May 2017 13:31:09 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Vince Weaver <vincent.weaver@...ne.edu>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Liang, Kan" <kan.liang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter

On Wed, May 24, 2017 at 9:01 AM, Vince Weaver <vincent.weaver@...ne.edu> wrote:
>
> On Wed, 24 May 2017, Andi Kleen wrote:
>
> > > Right, I did not even consider the rdpmc, but yeah, you will get a count that
> > > is not relevant to the user visible event. Unless you fake it using the time
> > > scaling fields there but that's ugly.
> >
> > Could add another scaling field to the mmap page for this.
>
> The whole point of the rdpmc() implementation is to be low overhead.
> If you have to parse 10 different mmap() fields it starts to defeat the
> purpose.
>
> I already have people really grumpy that you have to have one mmap() page
> per event, meaning you sacrifice one TLB entry for each event you are
> measuring.
>
>
> If the watchdog counter is constantly running, can't you just modify
> perf_event to just grab start/stop values at context switch time and
> provide the difference to the user?  Sort of like the "always running"
> patchsets that float around? Though I guess that doesn't help much with
> sampling.
>
This goes back to my initial point of simply increasing the period to
avoid false positive and stay with the
core-cycle event. It is much simpler. And again, if you are
deadlocked, it should not matter if you detect
it after 2mn or 5mn or 10mn. The accuracy is not so critical. And by
choosing a large enough period, you
avoid the issue with Turbo, even if you consider Turbo being able to
double your reference frequency.

Ultimately, I would like to see the watchdog move out of the PMU. That
is the only sensible solution.
You just need a resource able to interrupt on NMI or you handle
interrupt masking in software as has
been proposed on LKML.