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

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Message-ID: <20160120143117.GA22723@lerouge>
Date:	Wed, 20 Jan 2016 15:31:19 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	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 10:03:32AM +0100, Thomas Gleixner wrote:
> On Wed, 13 Jan 2016, Frederic Weisbecker wrote:
> > 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.
> 
> That's not a solution. That's just crap.

Have you seen the "RFC"? That's what we use when we are not yet confident
with a solution but we want to start a debate in order to find a proper one.

> 
> I tell you since years, that you need to fix that remote accounting stuff,
> but no, you insist on adding more trainwrecks left and right.

The solution you proposed to me was to do remote scheduler_tick() from
CPU 0 and this was nacked by peterz (and he was right).

We all know that we need to fix this remote accounting stuff, but I'm the
only one who actually _tries_, at least through RFC's to start discussions, such that I
find the right direction to move forward.

You're not helping me _at all_ with your shitty rants, all you're doing is discouraging me
and pushing me out to quit kernel development. I seriously thought about it but that's not
going to happen, unless there is a collective opinion toward the fact I'm a nuisance for the
community.

So go to hell Thomas!

> 
> > The problem with doing this remotely is that we can miss past cpu loads if
> > there was several enqueue/dequeue operations happening while tickless.
> 
> That's complete bullshit.
> 
> 1) How is remote accounting that happens every tick different from local
>    accounting which happens every tick?

Enqueue/dequeue don't happen on tick, unless there is a wakeup on that interrupt.

> 
> 2) How do you have enqueue/dequeue operations when you are running in full
>    nohz, i.e. one task is consuming 100% cpu time in user space?

Well that task is going to sleep, wake up, sleep like any other task. We need to
account these slices properly. If a second task wakes up and restart the tick, we must
make sure that the previous tickless frame got accounted properly.

Besides, if a SCHED_FIFO task runs (tickless) with SCHED_NORMAL tasks in the runqueue,
those are typically still accounted with the tick, so perhaps we need to keep that behaviour
without the tick as well and account those SCHED_NORMAL task's load.

> 
> I'm really tired of that tinkering. The proper solution is to make NOHZ_FULL
> depend on BROKEN.

Sure, knock yourself out.

> 
> Thanks,
> 
> 	tglx