linux-kernel - Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task enqueue/dequeue

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Date:	Wed, 20 Jan 2016 10:09:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task
 enqueue/dequeue

On Tue, Jan 19, 2016 at 06:03:19PM +0100, Frederic Weisbecker wrote:
> On Tue, Jan 19, 2016 at 02:17:08PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 13, 2016 at 05:01:31PM +0100, Frederic Weisbecker wrote:
> > > The full nohz CPU load is currently accounted on tick restart only.
> > > But there are a few issues with this model:
> > > 
> > > _ On tick restart, if cpu_load[0] doesn't contain the load of the actual
> > >   tickless load that just ran, we are going to account a wrong value.
> > >   And it is very likely to be so given that cpu_load[0] doesn't have
> > >   an opportunity to be updated between tick stop and tick restart.
> > > 
> > > _ If the runqueue had updates that didn't trigger a tick restart, we
> > >   are going to miss those CPU load changes.
> > > 
> > > A solution to fix this is to update the CPU load everytime we enqueue
> > > or dequeue a task in the fair runqueue and more than a jiffy occured
> > > since the last update.
> > 
> > Would not a much better solution be to do this remotely instead of from
> > one of the hottest functions in the scheduler?
> 
> The problem with doing this remotely is that we can miss past cpu loads if
> there was several enqueue/dequeue operations happening while tickless.

Its a timer based sample, it _always_ and per definition misses
intermediate state.

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);
	}

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.