Re: [PATCH 3/4] sched: introduce synchronized idle injection

From: Peter Zijlstra
Date: Wed Nov 18 2015 - 07:50:31 EST

On Wed, Nov 18, 2015 at 12:27:04PM +0000, Morten Rasmussen wrote:
> On Wed, Nov 18, 2015 at 11:35:41AM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 18, 2015 at 09:36:22AM +0100, Ingo Molnar wrote:
> > > What will such throttling do to latencies, as observed by user-space tasks? What's
> > > the typical expected frequency of the throttling frequency that you are targeting?
> >
> > The default has 5ms (iirc) of forced idle, so depending on what you do,
> > noticeable to outright painful.
> IIUC, it is 5 ticks, not ms.

The code uses hrtimers (badly), this means there _should_ not be a tick

> Which raises the question, doesn't that mean that we get disturbed four
> times on each cpu during the forced idle period? So idle injection only
> makes sense if the platform has package states with a target residency
> less than a jiffy. Or, do we enter NOHZ idle? I haven't looked closely
> enough to figure out yet.

The idea is to hit NOHZ.
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