linux-kernel - Re: [patch 2/2] x86,pat Convert memtype_lock into an rw

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Date:	Wed, 24 Mar 2010 17:36:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, "holt@....com" <holt@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [patch 2/2] x86,pat Convert memtype_lock into an rw_lock.

On Wed, 2010-03-24 at 09:12 -0700, Suresh Siddha wrote:
> On Wed, 2010-03-24 at 04:32 -0700, Peter Zijlstra wrote:
> > On Wed, 2010-03-17 at 16:19 -0800, Suresh Siddha wrote:
> > > On Wed, 2010-03-17 at 12:51 -0700, H. Peter Anvin wrote:
> > > > Well, as you know :) tglx and I are on the road ... I'll try to get to it on Friday before I take off again.
> > > 
> > > Also I talked to Thomas about this rwlock conversion and he referred to
> > > RT issues with rwlock. And the best is to avoid this using RCU.
> > 
> > Its not just RT, even for mainline rwlock_t is a massive pain and often
> > is no better (actually worse) than a spinlock due to the massive
> > cacheline bouncing it introduces.
> 
> Don't we have the same cacheline bouncing issues with the ticket
> spinlocks?

Sure, but the rwlock_t is unfair and can degrade into much worse
performance than the spinlock.

Thing is, rwlock_t needs to write to the cacheline for each read
acquire, so unless the hold time is much-much longer than the cacheline
bounce time, its just not worth it, but since its a rwlock_t it should
be have short hold time, hence its a useless construct :-)


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