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

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Message-ID: <20160120145416.GB22723@lerouge>
Date:	Wed, 20 Jan 2016 15:54:19 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Wed, Jan 20, 2016 at 10:09:06AM +0100, Peter Zijlstra wrote:
> 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.

Sure, but the problem is when these intermediate states are long enough.

> 
> 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? 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).

> 
> 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.

If the user makes use of full dynticks for soft isolation (for performance, can live
with a few interrupts...), there can be short moments of multitasking.