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

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Date:   Tue, 30 May 2017 09:39:59 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vince Weaver <vincent.weaver@...ne.edu>,
        Andi Kleen <ak@...ux.intel.com>,
        "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 Tue, May 30, 2017 at 2:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, May 28, 2017 at 01:31:09PM -0700, Stephane Eranian wrote:
>> 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.
>
> So even if we do the soft masking, we still need to deal with regions
> where the interrupts are disabled. Once an interrupt hits the soft mask
> we still hardware mask.
>
What I was thinking is that you never hardware mask, software always
catches the hw interrupts and keeps them pending or deliver them
depending on sw mask.

> So to get full and reliable coverage we still need an NMI source.
>
> I agree that it would be lovely to free up the one counter though.
>
>
> One other approach is running the watchdog off of _any_ PMI, then all we
> need to ensure is that PMIs happen semi regularly. There are two cases
> where this becomes 'interesting':
>
>  - we have only !sampling events; in this case we have PMIs but at the
>    max period to properly account for counter overflow. This is too
>    large a period. We'd have to muck with the max period of at least one
>    counter.
>
>  - we have _no_ events; in this case we need to somehow schedule an
>    event anyway.
>
> It might be possible to deal with both cases by fudging the state of one
> of the fixed counters. Never clear the EN bit for that counter and
> reduce the max period for that one counter.
>
>
> I think a scheme like that was mentioned before, but I'm also afraid
> that it'll turn into quite the mess if we try it. And by its very nature
> it adds complexity and therefore risks reducing the reliability of the
> thing :/