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Message-ID: <1287491931.16971.358.camel@gandalf.stny.rr.com>
Date: Tue, 19 Oct 2010 08:38:51 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Rakib Mullick <rakib.mullick@...il.com>,
LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
Gregory Haskins <ghaskins@...ell.com>
Subject: Re: [PATCH] sched_rt: Removes extra checking for nr_cpus_allowed
when calling find_lowest_rq
On Tue, 2010-10-19 at 13:02 +0200, Peter Zijlstra wrote:
> > Remove the check for nr_cpus_allowed of an RT task before calling
> > find_lowest_cpu() from select_task_rq_rt function. Cause
> > find_lowest_cpu checks for rt.nr_cpus_allowed of a process.
> > It would've been nice, if we make the check before calling
> > find_lowest_cpu. But its called from other places, so keep the
> > nr_cpus_allowed check into find_lowest_cpu().
>
Yes it is redundant, but I'm wondering if we should modify
find_lowest_rq() just to short cut this case.
That is, instead of assigning all the variables on the stack, which I'm
thinking __get_cpu_var() may act like a barrier() where gcc can't
optimize. We should probably also do the following:
static int find_lowest_rq(struct task_struct *task)
{
struct sched_domain *sd;
- struct cpumask *lowest_mask = __get_cpu_var(local_cpu_mask);
- int this_cpu = smp_processor_id();
- int cpu = task_cpu(task);
+ struct cpumask *lowest_mask;
+ int this_cpu;
+ int cpu;
if (task->rt.nr_cpus_allowed == 1)
return -1; /* No other targets possible */
+ lowest_mask = __get_cpu_var(local_cpu_mask);
+ this_cpu = smp_processor_id();
+ cpu = task_cpu(task;
Then removing the redundant check may not hurt anything.
-- Steve
> > Signed-off-by: Rakib Mullick <rakib.mullick@...il.com>
> > ---
> >
> > --- 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)) {
> > + if (unlikely(rt_task(rq->curr))) {
> > int cpu = find_lowest_rq(p);
> >
> > return (cpu == -1) ? task_cpu(p) : cpu;
--
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