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

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Date:	Sat, 23 Aug 2008 21:55:38 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	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

Andi Kleen wrote:
> On Fri, Aug 22, 2008 at 11:35:46AM -0700, Jeremy Fitzhardinge wrote:
>   
>> 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?
>>     
>
> It's likely slower than no kmalloc because
> there will be more instructions executed, the question is just how much.
>
>   
>> (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.)
>>     
>
> Ah so it was already 25% slower even without kmalloc? I thought
> that was with already. That doesn't sound good. Any idea where that slowdown 
> comes from?

Just longer code path, I think.  It calls the generic
smp_call_function_mask(), which then does a popcount on the cpu mask
(which it needs to do anyway), sees only one bit set, and then punts to
the smp_call_function_single() path.  If we maintained a cpus_online
count, then we could fast-path the call to smp_call_function_single() in
the two core/cpu case more efficiently (would still need to scan the
mask to extract the cpu number).

Or alternatively, maybe it isn't actually worth special casing
smp_call_function_single() with a multi-queue smp_call_function_mask()
implementation?

    J
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