Messages in this thread | | | Date | Fri, 18 Nov 2016 12:22:23 -0800 | From | Vikram Mulukutla <> | Subject | Re: spin_lock behavior with ARM64 big.Little/HMP |
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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() >>
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Thanks, Vikram
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