Messages in this thread | | | Date | Mon, 10 Dec 2018 23:51:34 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] cpuidle: Add 'above' and 'below' idle state metrics |
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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@infradead.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.
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