Re: [PATCH 0/2] nohz_full: Offload task_tick to remote housekeeping cpus for nohz_full cpus

From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Vatika Harlalka <vatikaharlalka@gmail.com>,
	mingo@redhat.com, tglx@linutronix.de, rafael.j.wysocki@intel.com,
	schwidefsky@de.ibm.com, linux-kernel@vger.kernel.org,
	preetium@andrew.cmu.edu, preeti.murthy@gmail.com
Subject: Re: [PATCH 0/2] nohz_full: Offload task_tick to remote housekeeping cpus for nohz_full cpus
Date: Thu, 13 Aug 2015 17:36:15 +0200	[thread overview]
Message-ID: <20150813153614.GD29958@lerouge> (raw)
In-Reply-To: <20150813150545.GD16853@twins.programming.kicks-ass.net>

On Thu, Aug 13, 2015 at 05:05:45PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 13, 2015 at 02:44:02PM +0200, Frederic Weisbecker wrote:
> > On Thu, Aug 13, 2015 at 02:22:23PM +0200, Peter Zijlstra wrote:
> > > On Thu, Aug 13, 2015 at 02:55:36PM +0530, Vatika Harlalka wrote:
> > > > This patchset is for offloading task_tick() to a remote housekeeping
> > > > cpu. The larger aim is to stop ticks on nohz_full cpus. For this, extra
> > > > work must be done by housekeeping cpus. So, task_tick is called from a
> > > > delayed workqueue for nohz_full cpus and the work is requeued every second
> > > > for those nohz_full cpus whose ticks are stopped while they are busy. In
> > > > the rest of the cases it will lead to redundant accounting. To facilitate
> > > > this, a new function tick_nohz_remote_tick_stopped is added to indicate
> > > > whether ticks are stopped on a remote cpu.
> > > > Tick related code in core.c is moved to tick.c
> > > 
> > > *sigh* of course you didn't read what I've written on this topic..
> > 
> > What is it? Note Vatika wrote this after my suggestion, so if there is an issue,
> > I'm likely the responsible :-) But I don't recall you opposed to this solution.
> 
> *sigh* of course you _could_ all use Google yourselves.
> 
> Re-read: https://patches.linaro.org/28290/

Sorry, there were dozens of threads about this issue and I got a bit confused.

> 
> I see nothing like the stuff I asked for in here, on top it creates the
> stupid tick.c file.

Right. I initially thought that we should make sched_tick() just work with long delays.
Then tglx suggested the offline idea but I lost track about our conversation.

But yeah making that scheduler_tick() working with long delays sound much better. Certainly
much more work but that's a natural evolution after all. It should pay in longer term.

We can start with update_cpu_load_active() which only works with HZ frequency updates or
nohz idle zero load decay. Now I think that stuff is only used for load balancing. I had
hopes this thing could be removed. I think Alex Shin (IIRC) tried but the patchset didn't
make it.

Thanks.