linux-kernel - Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load

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Date:   Tue, 9 Mar 2021 07:46:11 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Michal Suchï¿½nek <msuchanek@...e.de>
Cc:     npiggin@...il.com, Davidlohr Bueso <dbueso@...e.de>,
        peterz@...radead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, paulus@...ba.org,
        longman@...hat.com, will@...nel.org
Subject: Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load_relaxed

On Tue, 09 Mar 2021, Michal Suchï¿½nek wrote:

>On Mon, Mar 08, 2021 at 05:59:50PM -0800, Davidlohr Bueso wrote:
>> 49a7d46a06c3 (powerpc: Implement smp_cond_load_relaxed()) added
>> busy-waiting pausing with a preferred SMT priority pattern, lowering
>> the priority (reducing decode cycles) during the whole loop slowpath.
>>
>> However, data shows that while this pattern works well with simple
>                                              ^^^^^^^^^^^^^^^^^^^^^^
>> spinlocks, queued spinlocks benefit more being kept in medium priority,
>> with a cpu_relax() instead, being a low+medium combo on powerpc.
>...
>>
>> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
>> index aecfde829d5d..7ae29cfb06c0 100644
>> --- a/arch/powerpc/include/asm/barrier.h
>> +++ b/arch/powerpc/include/asm/barrier.h
>> @@ -80,22 +80,6 @@ do {									\
>>	___p1;								\
>>  })
>>
>> -#ifdef CONFIG_PPC64
>Maybe it should be kept for the simple spinlock case then?

It is kept, note that simple spinlocks don't use smp_cond_load_relaxed,
but instead deal with the priorities in arch_spin_lock(), so it will
spin in low priority until it sees a chance to take the lock, where
it switches back to medium.

Thanks,
Davidlohr