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

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Message-ID: <YBHqeXPMILg+352D@hirez.programming.kicks-ass.net>
Date:   Wed, 27 Jan 2021 23:34:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander A Sverdlin <alexander.sverdlin@...ia.com>
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

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.

> 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>
> -- 
> 2.10.2
>