linux-kernel - Re: [PATCH] locking/qrwlock: Allow multiple spinning readers

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Message-ID: <20160401104119.GA3604@arm.com>
Date:	Fri, 1 Apr 2016 11:41:19 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <waiman.long@....com>, Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH] locking/qrwlock: Allow multiple spinning readers

On Fri, Apr 01, 2016 at 12:31:43PM +0200, Peter Zijlstra wrote:
> On Thu, Mar 31, 2016 at 06:12:38PM -0400, Waiman Long wrote:
> > >>However, if we allow a limited number of readers to spin on the
> > >>lock simultaneously, we can eliminates some of the reader-to-reader
> > >>latencies at the expense of a bit more cacheline contention and
> > >>probably more power consumption.
> > >So the embedded people might not like that much.
> > 
> > It could be. It is always a compromise.
> 
> So ARM is the only one that currently waits without spinning and could
> care; so Will might have an opinion. One 'solution' would be to make
> this an optional feature.

Well, perhaps we could built this using the cmp-and-wait structure we spoke
about a couple of months back. What happened with that? Is there something I
need to go implement for ARM/arm64?

Will