linux-kernel - Re: [PATCH 2/7] nohz: New tick dependency mask

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Message-ID: <20151202141109.GD14766@lerouge>
Date:	Wed, 2 Dec 2015 15:11:10 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/7] nohz: New tick dependency mask

On Wed, Dec 02, 2015 at 01:48:06PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 01, 2015 at 11:20:28PM +0100, Frederic Weisbecker wrote:
> > On Tue, Dec 01, 2015 at 09:41:09PM +0100, Peter Zijlstra wrote:
> > > On Fri, Nov 13, 2015 at 03:22:04PM +0100, Frederic Weisbecker wrote:
> > > > The tick dependency is evaluated on every IRQ. This is a batch of checks
> > > > which determine whether it is safe to stop the tick or not. These checks
> > > > are often split in many details: posix cpu timers, scheduler, sched clock,
> > > > perf events. Each of which are made of smaller details: posix cpu
> > > > timer involves checking process wide timers then thread wide timers. Perf
> > > > involves checking freq events then more per cpu details.
> > > > 
> > > > Checking these details asynchronously every time we update the full
> > > > dynticks state bring avoidable overhead and a messy layout.
> > > > 
> > > > Lets introduce instead tick dependency masks: one for system wide
> > > > dependency (unstable sched clock), one for CPU wide dependency (sched,
> > > > perf), and task/signal level dependencies. The subsystems are responsible
> > > > of setting and clearing their dependency through a set of APIs that will
> > > > take care of concurrent dependency mask modifications and kick targets
> > > > to restart the relevant CPU tick whenever needed.
> > > 
> > > Maybe better explain why we need the per task and per signal thingy?
> > 
> > I'll detail that some more in the changelog. The only user of the per
> > task/per signal tick dependency is posix cpu timer. I've been first
> > proposing a global tick dependency as soon as any posix cpu timer is
> > armed. 
> 
> > It simplified everything but some reviewers complained (eg:
> > some users might want to run posix timers on housekeepers without
> > bothering full dynticks CPUs). I could remove the per signal
> > dependency with dispatching it through all threads in the group each
> > time there is an update but that's the best I can think of.
> 
> Right, I remember some of that. Seems worth preserving these reasons.
> Maybe even in code comments near the definition of these things.

Agreed! I'll comment that some more.

Thanks.
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