Re: spin_lock behavior with ARM64 big.Little/HMP

[Vikram Mulukutla]

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