linux-kernel - Re: [PATCH v2] cpuidle: Add 'above' and 'below' idle state metrics

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Date:   Wed, 12 Dec 2018 10:57:15 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>, dsmythies@...us.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        ggherdovich@...e.cz, Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v2] cpuidle: Add 'above' and 'below' idle state metrics

On Wed, 12 Dec 2018 at 10:46, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Dec 11, 2018 at 10:51:48AM +0100, Rafael J. Wysocki wrote:
> > On Mon, Dec 10, 2018 at 11:51 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > Dunno; it could be cold cachelines, at which point it can be fairly
> > > expensive. Also, being stuck with API is fairly horrible if you want to
> > > 'fix' it.
> >
> > All of the cache lines involved should've been touched earlier in this
> > code path by the governor.  At least menu and the new one both touch
> > them.
> >
> > The API part I'm not too worried about.  I know it is useful and two
> > other people have told that to me already. :-)
>
> Like said on IRC; I mostly wanted to raise the issue of overhead due to
> stats and ABI -- it's something I've been bitten by too many times :/
>
> If you're confident you're hitting the same lines with the already
> extant accouning (time and usage) and you cannot make the whole lot
> conditional because of ABI (bah) then I'll not stand in the way here.
>
> I agree the numbers are useful, I'm just weary of overhead.

I tend to agree. So then why not move this to debugfs, it seems like
it's really there it belongs. No?

Kind regards
Uffe