linux-kernel - Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

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Message-ID: <20080822224016.GN6744@linux.vnet.ibm.com>
Date:	Fri, 22 Aug 2008 15:40:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather
	than rcu

On Fri, Aug 22, 2008 at 08:33:46PM +0200, Andi Kleen wrote:
> > Would it make sense to push the freed-by-RCU memory further up the
> > hierarchy, so that such memory is not mistaken for recently freed
> > hot-in-cache memory?
> 
> Right now my impression is that it is not well understood why
> the kmalloc makes the IPI that much slower. In theory a kmalloc
> shouldn't be all that slow, it's essentially just a 
> "disable interrupts; unlink object from cpu cache; enable interrupts"
> with some window dressing. kfree() is similar.
> 
> Does it bounce a cache line on freeing perhaps?

It is indeed easy to focus on the wrong area when attempting to
improve performance.  Done it many times myself...  :-/

							Thanx, Paul
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