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Date: Tue, 5 May 2015 14:09:46 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Xunlei Pang <xlpang@126.com>
Cc: linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Juri Lelli <juri.lelli@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        Xunlei Pang <pang.xunlei@linaro.org>
Subject: Re: [PATCH v2 1/2] sched/rt: Check to push task away when its
 affinity is changed
Message-ID: <20150505120946.GL21418@twins.programming.kicks-ass.net>
References: <1430826968-10251-1-git-send-email-xlpang@126.com>
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On Tue, May 05, 2015 at 07:56:07PM +0800, Xunlei Pang wrote:
> +++ b/kernel/sched/core.c
> @@ -4773,6 +4773,9 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>  
>  	cpumask_copy(&p->cpus_allowed, new_mask);
>  	p->nr_cpus_allowed = cpumask_weight(new_mask);
> +
> +	if (p->sched_class->post_set_cpus_allowed)
> +		p->sched_class->post_set_cpus_allowed(p);
>  }
>  
>  /*
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 8885b65..4176f33 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2280,6 +2280,20 @@ static void set_cpus_allowed_rt(struct task_struct *p,
>  	update_rt_migration(&rq->rt);
>  }
>  
> +static void post_set_cpus_allowed_rt(struct task_struct *p)
> +{
> +	struct rq *rq;
> +
> +	if (!task_on_rq_queued(p))
> +		return;
> +
> +	rq = task_rq(p);
> +	if (!task_running(rq, p) &&
> +	    p->nr_cpus_allowed > 1 &&
> +	    !test_tsk_need_resched(rq->curr))
> +		push_rt_tasks(rq);
> +}

Guys, this is disgusting. Please don't do these minimal effort hacks.

Either fix up all the classes with a trivial set_cpus_allowed() function
and make do_set_cpus_allowed() := p->sched_class->set_cpus_allowed().

Or just do the p->{nr_,}cpus_allowed assignments in
set_cpus_allowed_rt() and keep it all in the one callback.