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

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Message-ID: <YBKha2GRFWyYp+Lz@hirez.programming.kicks-ass.net>
Date:   Thu, 28 Jan 2021 12:35:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander 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 Thu, Jan 28, 2021 at 08:29:57AM +0100, Alexander Sverdlin wrote:
> 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.

Hardware that needs a store-buffer flush after release is highly suspect
and needs big and explicit comments. Not vague hints.