linux-kernel - Re: [regression] cross core scheduling frequency drop bisected to 0c313cb20732

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Date:	Mon, 11 Apr 2016 15:21:45 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Doug Smythies <dsmythies@...us.net>
Subject: Re: [regression] cross core scheduling frequency drop bisected to 0c313cb20732

On Mon, Apr 11, 2016 at 2:38 PM, Rik van Riel <riel@...hat.com> wrote:
> On Mon, 2016-04-11 at 05:04 +0200, Mike Galbraith wrote:
>> On Sun, 2016-04-10 at 16:24 -0400, Rik van Riel wrote:
>> >
>> > On Sun, 2016-04-10 at 17:39 +0200, Mike Galbraith wrote:
>> >
>> > >
>> > > Should the default idle state not then be governor
>> > > dependent?  When I
>> > > set gov=performance, I'm expecting box to go just as fast as it
>> > > can
>> > > go
>> > > without melting.  Does polling risk CPU -> lava conversion?
>> > Current CPUs can only have some cores run at full speed
>> > (turbo mode) if other cores are idling and/or running at
>> > lower speeds.
>> The real world is very unlikely to miss the prettier numbers I'm
>> grieving over one tiny bit.  Knowing that doesn't make giving them up
>> any easier though.. byebye cycles (sniff) ;-)
>
> I suspect your pipe benchmark could be very relevant to
> network performance numbers, too.
>
> I would like to go into polling a little bit more aggressively
> in a future kernel,

Agreed, but ->

> and I think we can get away with it if we
> teach the polling loop to exit after we have spent enough time
> there that the menu governor will pick HLT after a few timed
> out poll loops.

-> my concern about this approach is that it would add an artificial
point to the menu governor statistics at whatever the timeout is
chosen to be.