linux-kernel - Re: [PATCH 04/32] x86: New cpuset nohz irq vector

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Date:	Tue, 30 Oct 2012 20:07:53 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Subject: Re: [PATCH 04/32] x86: New cpuset nohz irq vector

On Wed, 2012-10-31 at 00:51 +0100, Frederic Weisbecker wrote:

> > Probably just use irq_work for self ipis, and normal ipis for other
> > CPUs.
> 
> Right. And that's one more reason why we want to know if the arch
> implements irq work with self ipis or not. If the arch can't, then we
> just don't stop the tick.

We can just allow certain archs to have cpuset/nohz. Make it depend on
features that you want (or makes nohz easier to implement).

> 
> > Also, what reason do we have to force a task out of nohz? IOW, do we
> > really need this?
> 
> When a posix CPU timer is enqueued, when a new task is enqueued, etc...

I was thinking about something other than itself. That is, who would
enqueue a posix cpu timer on the cpu other than the task running with
nohz on that cpu?

A new task would send the schedule ipi too. Which would enqueue the task
and take the cpu out of nohz, no?


> 
> >
> > Also, perhaps we could just tag onto the schedule_ipi() function instead
> > of having to create a new IPI for all archs?
> 
> irq work should be just fine. No need to add more overhead on the
> schedule ipi I think.

irq_work can send the work to another CPU right? This part I wasn't sure
about.

-- Steve


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