Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

[Peter Zijlstra]

On Tue, Jan 30, 2018 at 01:25:27PM +0000, Mel Gorman wrote:
> > Because, esp. in this scenario; a task migrating; the hardware really
> > can't do anything sensible, whereas the OS _knows_.
> 
> Potentially yes. One option without HWP would be to track utilisation
> for a task or artifically boost it for a short period after migration so
> a higher p-state is potentially selected. With HWP, a hint would have to
> be given to the hardware to try select a higher frequency but I've no idea
> how expensive that is or how it would behave on different implementations
> of HWP. It may also be a game of whack-a-mole trying to get every cpufreq
> configuration correct.

We have much of this infrastructure and have been working on improving
it [1]. We already track per task utilization and feed it into a cpufreq
governor (schedutil).

> One advantage of using fewer cores is that it should
> work regardless of cpufreq driver.

Sure, and I started out by saying this patch isn't necessarily bad; but
I think our 'use' [2] of HWP _is_.


[1] https://lkml.kernel.org/r/20180123180847.4477-1-patrick.bellasi@xxxxxxx
[2] IIRC we basically don't do _anything_ when HWP.