linux-kernel - Re: [PATCH 3/6] MIPS: Octeon: qspinlock: Flush write buffer

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Date:   Thu, 28 Jan 2021 08:29:57 +0100
From:   Alexander Sverdlin <alexander.sverdlin@...ia.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paul Burton <paul.burton@...tec.com>, linux-mips@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] MIPS: Octeon: qspinlock: Flush write buffer

Hi!

On 27/01/2021 23:34, Peter Zijlstra wrote:
> On Wed, Jan 27, 2021 at 09:36:24PM +0100, Alexander A Sverdlin wrote:
>> From: Alexander Sverdlin <alexander.sverdlin@...ia.com>
>>
>> Flushing the write buffer brings aroung 10% performace on the tight
>> uncontended spinlock loops on Octeon. Refer to commit 500c2e1fdbcc
>> ("MIPS: Optimize spinlocks.").
> No objection to the patch, but I don't find the above referenced commit
> to be enlightening wrt nudge_writes(). The best it has to offer is the
> comment that's already in the code.

My point was that original MIPS spinlocks had this write-buffer-flush and
it got lost on the conversion to qspinlocks. The referenced commit just
allows to see the last MIPS-specific implementation before deletion.

>> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...ia.com>
>> ---
>>  arch/mips/include/asm/spinlock.h | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/mips/include/asm/spinlock.h b/arch/mips/include/asm/spinlock.h
>> index 8a88eb2..0a707f3 100644
>> --- a/arch/mips/include/asm/spinlock.h
>> +++ b/arch/mips/include/asm/spinlock.h
>> @@ -24,6 +24,9 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
>>  	/* This could be optimised with ARCH_HAS_MMIOWB */
>>  	mmiowb();
>>  	smp_store_release(&lock->locked, 0);
>> +#ifdef CONFIG_CPU_CAVIUM_OCTEON
>> +	nudge_writes();
>> +#endif
>>  }
>>  
>>  #include <asm/qspinlock.h>

-- 
Best regards,
Alexander Sverdlin.