linux-kernel - Re: [PATCH] sched: correct how RT task is picked

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Date:	Wed, 18 May 2011 21:19:44 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched: correct how RT task is picked

On Wed, May 18, 2011 at 10:31 AM, Yong Zhang <yong.zhang0@...il.com> wrote:
> On Wed, May 18, 2011 at 9:38 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>> I think you can take a look at next_prio(), it just calculate the
>>> next highest task on the current cpu; in this case,
>>> cpumask_test_cpu(cpu, &p->cpus_allowed) will be true for the most
>>> of time, but maybe that task is bound to this cpu.
>>
>> I've been looking at the history here, and I think that '-1' is a relic.
>>
>> If you look at sched_rt.c in f65eda4f789168ba5ff3fa75546c29efeed19f58:
>>
>> $ git show f65eda4f:kernel/sched_rt.c
>>
>> You'll see that push_rt_task calls pick_next_highest_task_rt() with a
>> -1. That code has long been replaced.
>
> Yeah, the condition "cpu < 0" could be removed since we have no
> that kind of caller.
>
>>
>> I'm a bit nervous about taking Hillf's patch as is. But a little more
>> reviewing and testing may prove that it is legit.
>
> But another point is like I said before:
> 'cpumask_test_cpu(cpu, &p->cpus_allowed)' doesn't equal to
> 'p->rt.nr_cpus_allowed > 1' because we could have bounded
> task.
> So the condition 'if (cpumask_test_cpu(cpu, &p->cpus_allowed))'
> in Hillf's patch is not sufficient.
>

Hi all

The patch is prepared again, in which tests for both cpu and
nr_cpus_allowed are dropped.

The reason to drop nr_cpus_allowed is to make sure that the
returned value is correct for both case that cpu == rq->cpu and
case that cpu != rq->cpu.

thanks

Hillf
---

--- a/kernel/sched_rt.c	2011-04-27 11:48:50.000000000 +0800
+++ b/kernel/sched_rt.c	2011-05-18 21:16:22.000000000 +0800
@@ -1149,9 +1149,7 @@ static void deactivate_task(struct rq *r

 static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
 {
-	if (!task_running(rq, p) &&
-	    (cpu < 0 || cpumask_test_cpu(cpu, &p->cpus_allowed)) &&
-	    (p->rt.nr_cpus_allowed > 1))
+	if (!task_running(rq, p) && cpumask_test_cpu(cpu, &p->cpus_allowed))
 		return 1;
 	return 0;
 }
--
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