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Date: Fri, 22 Aug 2008 11:35:46 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Andi Kleen <andi@firstfloor.org>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Pekka Enberg <penberg@cs.helsinki.fi>, Ingo Molnar <mingo@elte.hu>,
       Nick Piggin <nickpiggin@yahoo.com.au>,
       "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>,
       Suresh Siddha <suresh.b.siddha@intel.com>,
       Jens Axboe <jens.axboe@oracle.com>,
       Rusty Russell <rusty@rustcorp.com.au>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than
 rcu
References: <48AE0883.6050701@goop.org> <20080822062800.GQ14110@elte.hu> <84144f020808220006n25d684b1n9db306ddc4f58c4c@mail.gmail.com> <48AEC6B2.1080701@linux-foundation.org> <20080822151156.GA6744@linux.vnet.ibm.com> <48AEF3FD.70906@linux-foundation.org> <20080822182915.GG6744@linux.vnet.ibm.com> <20080822183346.GS23334@one.firstfloor.org>
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Andi Kleen wrote:
> 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?

I think it's just an assumption that it would be slower.  Has anyone
measured it?

(Note: The measurements I posted do not cover this path, because it was
on a two cpu system, and it was always using the call-single path.)

    J