linux-kernel - Re: [RFC v2 3/7] Improve the tracking of active utilisation

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Message-ID: <20160405180050.GO3408@twins.programming.kicks-ass.net>
Date:	Tue, 5 Apr 2016 20:00:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	luca abeni <luca.abeni@...tn.it>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [RFC v2 3/7] Improve the tracking of active utilisation

On Tue, Apr 05, 2016 at 07:56:57PM +0200, luca abeni wrote:
> > > +	if (rq != cpu_rq(cpu)) {
> > 
> > I don't think this is right, you want:
> > 
> > 	if (task_cpu(p) != cpu) {
> > 
> > because @cpu does not need to be task_cpu().
> Uhm... I must have misunderstood something in the code, then :(
> What I want to do here is to check if select_task_rq_dl() selected
> a new CPU for this task... Since at the beginning of the function
> rq is set as
> 	rq = cpu_rq(cpu);
> I was thinkint about checking if this is still true (if not, it
> means that the value of "cpu" changed).
> 
> I'll look at it again.

Basically because:

  ac66f5477239 ("sched/numa: Introduce migrate_swap()")

we cannot (in general) assume .cpu == task_cpu(p).

Now it might still be true for deadline tasks, but I find it easier to
simply not rely on such assumptions.