linux-kernel - Re: [PATCH 0/2] Add test to validate udelay

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Date:	Wed, 07 May 2014 15:46:45 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Doug Anderson <dianders@...omium.org>
CC:	David Riley <davidriley@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH 0/2] Add test to validate udelay

On 05/07/2014 11:32 AM, Doug Anderson wrote:
> John,
>
> On Wed, May 7, 2014 at 11:10 AM, John Stultz <john.stultz@...aro.org> wrote:
>> On 05/06/2014 09:19 PM, Doug Anderson wrote:
>>> John,
>>>
>>> On Tue, May 6, 2014 at 5:25 PM, John Stultz <john.stultz@...aro.org> wrote:
>>>> Really, I'm curious about the backstory that made you generate the test?
>>>> I assume something bit you where udelay was way off? Or were you using
>>>> udelay for some sort of accuracy sensitive use?
>>> Several times we've seen cases where udelay() was pretty broken with
>>> cpufreq if you were actually implementing udelay() with
>>> loops_per_jiffy.  I believe it may also be broken upstream on
>>> multicore systems, though now that ARM arch timers are there maybe we
>>> don't care as much?
>>>
>>> Specifically, there is a lot of confusion between the global loops per
>>> jiffy and the per CPU one.  On ARM I think we always use the global
>>> one and we attempt to scale it as cpufreq changes.  ...but...
>>>
>>> * cores tend scale together and there's a single global.  That means
>>> you might have started the delay loop at one freq and ended it at
>>> another (if another CPU changes the freq).
>> Good point. The loops based delay would clearly be broken w/ ASMP unless
>> we use per-cpu values that are scaled(and as you point out, we don't
>> scale the value mid-delay). Time based counters for udelay() - like the
>> arch timer you mention - are a much better way to work around this.
> Locally we have this:
> * https://chromium-review.googlesource.com/189885
>   ARM: Don't ever downscale loops_per_jiffy in SMP systems
>
> I didn't think upstream would really want this given the move to arch
> timers, but I'm happy to post it.

Might be good just to get the discussion going. I agree its probably not
the best solution, and likely the timer delay is the right path, but but
I think we need to have some rails in place so that other folks don't
trip over this.

Maybe a combination of your change and something where on systems that
see cpufreq changes (or cores with different frequencies) complain
loudly if they're not configured to use the delay timer?


>>> * I believe there's some strange issues in terms of how the loops per
>>> jiffy variable is initialized and how the "original CPU freq" is.  I
>>> know we ran into issues on big.LITTLE where the LITTLE cores came up
>>> and clobbered the loops_per_jiffy variable but it was still doing math
>>> based on the big cores.
>> Hrm. I don't have a theory on this right now, but clearly there are
>> issues to be resolved, so having your tests included would be a good
>> thing to help find these issues.
> Locally we added:
> * https://chromium-review.googlesource.com/189725
>   init: Don't decrease loops_per_jiffy when a CPU comes up
>
> ...and that "fixed" things for us.  Specifically what happened was:
>
> - A15s boot up at 1.8GHz (though they can actually go up to 1.9/2.0).
>
> - A7s boot up at ~600MHz and clobbered loops_per_jiffy with something tiny.
>
> - In the first "cpufreq_callback" in "arch/arm/kernel/smp.c", we
> stored the current _A15_ frequency upon the first cpufreq transition
> and the current _A7_ loops per jiffy (since the A7s clobbered it).
>
>
> It seemed like our situation was so messed up that upstream probably
> wouldn't want the fix (using loops_per_jiffy in an HMP system is
> pretty insane), but again I'm happy to send it up.

Yea. Again, might be good to send it out just so more folks are aware
(particularly outside the arm world) that ASMP systems are here and the
generic delay infrastructure has assumptions that are not compatible.

thanks
-john

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