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

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Date:   Mon, 10 Dec 2018 23:51:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Doug Smythies <dsmythies@...us.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v2] cpuidle: Add 'above' and 'below' idle state metrics

On Mon, Dec 10, 2018 at 10:36:40PM +0100, Rafael J. Wysocki wrote:
> On Mon, Dec 10, 2018 at 1:21 PM Peter Zijlstra <peterz@...radead.org> wrote:

> > One question on this; why is this tracked unconditionally?
> 
> Because I didn't quite see how to make that conditional in a sensible way.

Something like:

	if (static_branch_unlikely(__tracepoint_idle_above) ||
	    static_branch_unlikely(__tracepoint_idle_below)) {

		// do stuff that calls trace_idle_above() /
		// trace_idle_below().

	}

> These things are counters and counting with the help of tracepoints
> isn't particularly convenient (and one needs debugfs to be there to
> use tracepoints and they require root access etc).

Root only should not be a problem for a developer; and aren't these
numbers only really interesting if you're prodding at the idle governor?

> > Would not a tracepoint be better?; then there is no overhead in the
> > normal case where nobody gives a crap about these here numbers.
> 
> There is an existing tracepoint that in principle could be used to
> produce this information, but it is such a major PITA in practice that
> nobody does that.  Guess why. :-)

Sounds like you need to ship a convenient script or something :-)

> Also, the "usage" and "time" counters are there in sysfs, so why not these two?
> 
> And is the overhead really that horrible?

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.