linux-kernel - Re: [PATCH] ARM: Don't ever downscale loops_per

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Message-ID: <CAD=FV=X4h0nfO7a10NXe+d76xj5ttBJx=q1pAvW4aPzrc22BoQ@mail.gmail.com>
Date:	Thu, 8 May 2014 11:06:24 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Russell King <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	John Stultz <john.stultz@...aro.org>,
	David Riley <davidriley@...omium.org>,
	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Richard Zhao <richard.zhao@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Shawn Guo <shawn.guo@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Stephen Warren <swarren@...dia.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems

Nicolas,

On Thu, May 8, 2014 at 10:43 AM, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Thu, 8 May 2014, Doug Anderson wrote:

>> Longer delays aren't very good, but IMHO having some delays of 100 =>
>> 1000 is better than having delays of 100 => 75.  The former will cause
>> mostly performance problems and the later will cause real correctness
>> problems.
>> I'm not saying that 100 => 1000 is good, it's just less bad.
>
> There might be some cases where precise timing is needed though.
> I thought I came across one such case in the past but I can't remember
> which.

If precise timing is needed, shouldn't it be using ktime?


>> I will make the argument that this patch makes things less broken
>> overall on any systems that actually end up running this code, but if
>> you want NAK it then it won't cause me any heartache.  ;)
>
> What I insist on is for this issue to be solved using a stable counter
> such a timer when available.  It _is_ available on one of the target you
> mentioned so that is the solution you should add to your tree.

Yup, we're working on it.


> Investigating a similar solution for your other target should be
> preferred to hacking the udelay loop. This way you're guaranteed to
> solve this problem fully.

I have no other target in mind.  I'm merely sending this up there just
in case there is some cpufreq running ARM board that is SMP and has no
timer-based udelay.  Those are the only boards that could possibly be
running this code anyway.

I guess I would say that my patch is unhacking the this code.  The
code after my patch is simpler.  I would perhaps argue that (ec971ea
ARM: add cpufreq transiton notifier to adjust loops_per_jiffy for smp)
should never have landed to begin with.

-Doug
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