linux-kernel - Re: Performance regression in v3.14

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Date:	Wed, 28 May 2014 09:59:45 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Dirk Brandewie <dirk.brandewie@...il.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Johan Hovold <jhovold@...il.com>, dirk.j.brandewie@...el.com,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Doug Smythies <dsmythies@...us.net>,
	Yuyang Du <yuyang.du@...el.com>,
	Stratos Karafotis <stratosk@...aphore.gr>
Subject: Re: Performance regression in v3.14

[ +CC: Greg, Doug, Stratos, Yuyang ]

On Wed, May 21, 2014 at 11:00:51AM +0200, Johan Hovold wrote:
> On Wed, May 07, 2014 at 07:10:49AM -0700, Dirk Brandewie wrote:
> > On 05/06/2014 10:40 PM, Viresh Kumar wrote:
> > > Cc'ing Dirk who is taking care of intel-pstate driver.
> > >
> > 
> > Thanks Viresh I had seen this thread.
> > 
> > I am looking into it
> 
> Any updates on this, Dirk? 3.14 is still basically unusable with the
> intel_pstate driver.
> 
> Any fixes or workarounds posted elsewhere that I can apply in the
> meantime?

Another week and still no reply, Dirk?

I tried applying your (rejected) patch "intel_pstate: Remove C0
tracking" posted here:

	https://lkml.org/lkml/2014/5/8/574

to v3.14.4 and it fixes the problem as expected.

So we have a commit fcb6a15c2e7e ("intel_pstate: Take core C0 time into
account for core busy calculation") that went into v3.14-rc2 (and was
even marked for *stable*) that first broke Greg KH's system:

	https://lkml.org/lkml/2014/2/19/626

That was apparently fixed by e66c17683746 ("intel_pstate: Change
busy calculation to use fixed point math."), but still left v3.14
basically unusable for lower-intensity workloads such as my
bash-completion example and other reported regressions:

	https://bugzilla.kernel.org/show_bug.cgi?id=75121

Sure there may be issues with v3.13 not hitting the lowest frequencies
but at least the system was *usable*.

In my opinion there's really no other option than to restore the 3.13
behaviour by effectively reverting fcb6a15c2e7e ("intel_pstate: Take
core C0 time into account for core busy calculation") until you have
figured out a way to take C0 into account without breaking things too
badly.

Johan
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