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

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Date:	Mon, 3 Aug 2015 11:59:07 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.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 07/31/2015 10:49 AM, Frederic Weisbecker wrote:
> Instead of doing a per signal dependency, I'm going to use a per task
> one. Which means that if a per-process timer is enqueued, every thread
> of that process will have the tick dependency. But if the timer is
> enqueued to a single thread, only the thread is concerned.
>
> We'll see if offloading becomes really needed. It's not quite free because
> the housekeepers will have to poll on all nohz CPUs at a Hz frequency.

Seems reasonable for now!

Why would we need the Hz frequency polling, though?  I would
think it should be possible to just arrange it such that the timer
for posix cpu timers would just always be placed either on the core
that requested it, or if that core is nohz_full, on a housekeeping
core.  Then it would eventually fire from the housekeeping core,
and the logic could be such that (for a process-wide timer) it
would preferentially interrupt threads from that process that
were running on the housekeeping cores.  No polling.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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