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

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Date:	Thu, 30 Jul 2015 15:52:54 -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 07/30/2015 03:45 PM, Frederic Weisbecker wrote:
>
>>>> You mentioned needing two fields, for task and for process, but in
>>>> fact let's just add the one field to the one thing that needs it and
>>>> not worry about additional possible future needs.  And note that it's
>>>> the task_struct->signal where we need to add the field for posix cpu
>>>> timers (the signal_struct) since that's where the sharing occurs, and
>>>> given CLONE_SIGHAND I imagine it could be different from the general
>>>> "process" model anyway.
>>> Well, posix cpu timers can be install per process (signal struct) or
>>> per thread (task struct).
>>>
>>> But we can certainly simplify that with a per process flag and expand
>>> the thread dependency to the process scope.
>>>
>>> Still there is the issue of telling the CPUs where a process runs when
>>> a posix timer is installed there. There is no process-like tsk->cpus_allowed.
>>> Either we send an IPI everywhere like we do now or we iterate through all
>>> threads in the process to OR all their cpumasks in order to send that IPI.
>> Is there a reason the actual timer can't run on a housekeeping
>> core?  Then when it does wake_up_process() or whatever, the
>> specific target task will get an IPI to wake up at that point.
> It makes sense if people run posix cpu timers on nohz full CPUs. But nobody
> reported such usecase yet.

The corner case I was trying to address with my comment above
is when a process includes both housekeeping and nohz_full threads.
This is generally a bad idea in my experience, but our customers
do this sometimes (usually because they're porting a big pile of
code from somewhere else), and if so it would be good if we didn't
have to keep every thread in that task ticking; presumably it is
enough to ensure the timer lands on a housekeeping core instead,
possibly the one for the non-fast-path thread in question, and then
the regular IPIs from wake_up_process() will be sufficient if for
some lame reason the signal ends up handled on a nohz_full core.

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

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