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 [off-list ref] wrote:

quoted

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?

quoted

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.