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Date: Wed, 20 Jan 2016 17:56:57 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Byungchul Park <byungchul.park@lge.com>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Christoph Lameter <cl@linux.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Mike Galbraith <efault@gmx.de>, Rik van Riel <riel@redhat.com>
Subject: Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task
 enqueue/dequeue
Message-ID: <20160120165657.GN6357@twins.programming.kicks-ass.net>
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On Wed, Jan 20, 2016 at 03:54:19PM +0100, Frederic Weisbecker wrote:

> > You can simply do:
> > 
> > 	for_each_nohzfull_cpu(cpu) {
> > 		struct rq *rq = rq_of(cpu);
> > 
> > 		raw_spin_lock(&rq->lock);
> > 		update_cpu_load_active(rq);
> > 		raw_spin_unlock(&rq->lock);
> > 	}
> 
> But from where should we do that? 

house keeper thingy

> Maybe we can do it before we call source/target_load(), on the
> selected targets needed by the caller? The problem is that if we do
> that right after a task got enqueued on the nohz runqueue, we may
> accidentally account it as the whole dynticks frame (I mean, if we get
> rid of that enqueue/dequeue accounting).

Yes so? What if the current tick happens right after a task get
enqueued? Then we account the whole tick as !idle, even tough we might
have been idle for 99% of the time.

Not a problem, this is sampling.

Doing it locally or remotely doesn't matter.

> > Also, since when can we have enqueues/dequeues while NOHZ_FULL ? I
> > thought that was the 1 task 100% cpu case, there are no
> > enqueues/dequeues there.
> 
> That's the most optimized case but we can definetly have small moments
> with more than one task running. For example if we have a workqueue,
> or such short and quick tasks.

The moment you have nr_running>1 the tick comes back on.