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

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Message-ID: <1460378295.25336.5.camel@redhat.com>
Date:	Mon, 11 Apr 2016 08:38:15 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	"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, 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, 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.

That way while we run a workload that actually benefits from
polling, we will get polling, but once we run a workload that
actually sleeps longer than the HLT threshold, we will quickly
fall back to HLT.

With 10Gbps network traffic, it could make a real difference
whether or not the CPU can wake up immediately, or takes a
microsecond to wake up...

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