linux-kernel - Re: [PATCH] sched_rt: Removes extra checking for nr_cpus_allowed when calling find_lowest

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Date:	Tue, 19 Oct 2010 06:57:12 -0600
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Rakib Mullick" <rakib.mullick@...il.com>,
	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched_rt: Removes extra checking for
 nr_cpus_allowed when calling find_lowest_rq

>>> On 10/19/2010 at 07:02 AM, in message <1287486167.1994.1.camel@...ns>, Peter
Zijlstra <peterz@...radead.org> wrote: 
> On Tue, 2010-10-19 at 16:57 +0600, Rakib Mullick wrote:

[snip]

>> --- linus-rc8/kernel/sched_rt.c	2010-10-15 05:26:43.000000000 +0600
>> +++ rakib-rc8/kernel/sched_rt.c	2010-10-19 16:22:30.000000000 +0600
>> @@ -971,8 +971,7 @@ select_task_rq_rt(struct rq *rq, struct
>>  	 * that is just being woken and probably will have
>>  	 * cold cache anyway.
>>  	 */
>> -	if (unlikely(rt_task(rq->curr)) &&
>> -	    (p->rt.nr_cpus_allowed > 1)) {

I think the motivation here was that checking nr_cpus_allowed is far cheaper than taking the hit on a function call in this particularly hot path.  As Steven points out in a follow-up reply, the function call has additional overhead before the equivalent check is made again.  We could possibly optimize this with some of the suggestions he made, but I am not sure if it is worth it (alone) as the call overhead would still be present.  OTOH, the cases where nr_cpus_allowed <= 1 are probably rare in the grand scheme of things.

My opinion is the check should probably remain (if but perhaps with a comment to explain its existence) unless someone (Rakib, hint hint) is willing to do some benchmarking to demonstrate that it doesn't actually have any positive impact.  It probably also makes sense to take Steve's suggested changes to improve the places that use the function without external optimization.

Kind Regards,
-Greg

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