linux-kernel - Re: [PATCH 08/10] posix-cpu-timers: Migrate to use new tick dependency mask model

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Message-ID: <20150803173936.GC26022@lerouge>
Date:	Mon, 3 Aug 2015 19:39:37 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.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 08/10] posix-cpu-timers: Migrate to use new tick
 dependency mask model

On Mon, Aug 03, 2015 at 07:12:43PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 31, 2015 at 04:49:55PM +0200, Frederic Weisbecker wrote:
> > Instead of doing a per signal dependency, I'm going to use a per task
> > one.
> 
> Urgh, does this mean you'll keep the horrid tick_nohz_task_switch()
> thing?

I thought I would drop it, but now that I think about it more, I think I
need to keep it because if we enqueue a posix timer to a sleeping task
and that task later wakes up, we must restart the tick, and that is only
possible with a check on context switch :-(

This current patchset removed the need for that with a global dependency
for posix timers: as long as there is one enqueued we keep the tick. But
Chris and Luiz fear that Tilera users have posix timers on housekeepers.
They also suggested we offline the posix timers. I fear it's going to be
a high overhead as it means polling on the target task context of execution.
Unless we move the task itself to housekeepers...
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