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SubjectRe: spin_lock behavior with ARM64 big.Little/HMP

Hi Sudeep,

Thanks for taking a look!

On 2016-11-18 02:30, Sudeep Holla wrote:
> Hi Vikram,
>
> On 18/11/16 02:22, Vikram Mulukutla wrote:
>> Hello,
>>
>> This isn't really a bug report, but just a description of a
>> frequency/IPC
>> dependent behavior that I'm curious if we should worry about. The
>> behavior
>> is exposed by questionable design so I'm leaning towards don't-care.
>>
>> Consider these threads running in parallel on two ARM64 CPUs running
>> mainline
>> Linux:
>>
>
> Are you seeing this behavior with the mainline kernel on any platforms
> as we have a sort of workaround for this ?
>

If I understand that workaround correctly, the ARM timer event stream is
used
to periodically wake up CPUs that are waiting in WFE, is that right? I
think
my scenario below may be different because LittleCPU doesn't actually
wait
on a WFE event in the loop that is trying to increment lock->next, i.e.
it's
stuck in the following loop:

ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
" prfm pstl1strm, %3\n"
"1: ldaxr %w0, %3\n"
" add %w1, %w0, %w5\n"
" stxr %w2, %w1, %3\n"
" cbnz %w2, 1b\n",


I have been testing internal platforms; I'll try to test on something
available publicly that's b.L. In any case, the timer event stream was
enabled
when I tried this out.

>> (Ordering of lines between the two columns does not indicate a
>> sequence of
>> execution. Assume flag=0 initially.)
>>
>> LittleARM64_CPU @ 300MHz (e.g.A53) | BigARM64_CPU @ 1.5GHz (e.g.
>> A57)
>> -------------------------------------+----------------------------------
>> spin_lock_irqsave(s) | local_irq_save()
>> /* critical section */
>> flag = 1 | spin_lock(s)
>> spin_unlock_irqrestore(s) | while (!flag) {
>> | spin_unlock(s)
>> | cpu_relax();
>> | spin_lock(s)
>> | }
>> | spin_unlock(s)
>> | local_irq_restore()
>>

[...]

Thanks,
Vikram

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