Re: [PATCH 5/8] sched/cpufreq: pass sched class into cpufreq_update_util

From: Rafael J. Wysocki
Date: Wed Mar 16 2016 - 08:39:17 EST

On Wed, Mar 16, 2016 at 9:53 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Mar 16, 2016 at 09:29:59AM +0100, Vincent Guittot wrote:
>> I wonder if it's really worth passing per sched_class request to
>> sched_util ? sched_util is about selecting a frequency based on the
>> utilization of the CPU, it only needs a value that reflect the whole
>> utilization. Can't we sum (or whatever the formula we want to apply)
>> utilizations before calling cpufreq_update_util
> So I've thought the same; but I'm conflicted, its a shame to compute
> anything if the call then doesn't do anything with it.
> And keeping a structure of all the various numbers to pass in also has
> cost of yet another cacheline to touch.

In principle we can use high-order bits of util and max to encode the
information on where they come from.

Of course, that translates to additional ifs in the governor, but I
guess they are unavoidable anyway.