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

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Message-ID: <20160401105457.GO3430@twins.programming.kicks-ass.net>
Date:	Fri, 1 Apr 2016 12:54:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
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 11:41:19AM +0100, Will Deacon wrote:
> 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?

Ah, yes, I forgot about that. Lemme go find that discussions and see
what I can do there.