udelay() broken for SMP cores?

[skannan at codeaurora.org]

> On Tue, Apr 20, 2010 at 11:43:23PM -0700, Saravana Kannan wrote:
>>
>>>> I looked at arch/arm/lib/delay.S and it looks like __udelay and
>>>> __const_udelay won't work correctly for SMP cores. The code just uses
>>>> the loops_per_jiffy variable instead of the per-CPU loops per jiffy
>>>> data.
>>>>
>>>> Is anyone already working on a fix for that? If not, I can fix it up
>>>> in
>>>> a way that's hopefully palatable to the community.
>>>
>>
>>> If you have case where individual CPUs are running at different speed
>>> and different
>>> Tick rate then only this can make difference.
>>
>> Yes. I was talking about the case where the CPUs could be running at
>> different speeds.
>
> We don't support that; if we did, we'd have to disable preempt for every
> call to mdelay/udelay to ensure that the thread is locked to a particular
> CPU.  I suspect that will (a) destroy RT scheduling preformance (b)
> increase preempt latency to an undesirable extent.
>

Is this an ARM specific decision? Cpufreq certainly supports per cpu scaling
and x86 udelay uses per-CPU data. So your concern should apply for x86
too. I had the same concern and was planning on bring it up in the cpufreq
mailing list after I made sure I didn't misunderstand anything.

Btw, your concern should apply for single core scaling too, right? Context
switch can complete within max udelay (general - 5ms, ARM - 2ms) time and
CPU could have jumped
from lowest to highest speed in that time and mess up udelay. I didn't see
any code in cpufreq that deferred scaling during udelay. So, that's something
I plan to ask cpufreq folks too.

Let me know what you think.

Thanks,
Saravana
P.S: Sent from phone. Pls excuse formatting issues.