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

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Message-ID: <20160121132525.GA5838@lerouge>
Date:	Thu, 21 Jan 2016 14:25:28 +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 07:25:03PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 20, 2016 at 06:21:07PM +0100, Frederic Weisbecker wrote:
> 
> > You mean a periodic call to the above from the housekeepers?
> > 
> > I didn't think about doing that because you nacked that approach with
> > scheduler_tick(). This isn't much different.
> 
> This does _one_ thing, namely load accounting. scheduler_tick() does a
> whole bunch of things, some which really do not make much sense to do
> remotely.

Ok.

> 
> > It means the housekeeper is entirely dedicated to full dynticks CPUs.
> 
> Depends on how many there are, and how many housekeepers. I would
> suggest one housekeeper cpu per node or so to make sure this keeps
> working.

That's possible to do.

> 
> Then again, these load values aren't super important, esp. not for the
> nohz_full case where you typically don't care about load balancing.

In this case maybe we can assume that these stats don't need precision at
all on nohz full and when the full nohz frame starts, we define
all cpu_load[i] = cpu_load[0] = weighted_cpuload(cpu) and we keep it that way until
the nohz full frame ends. This way we avoid the remote update.