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

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Date:	Fri, 31 Jul 2015 16:49:55 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.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 Thu, Jul 30, 2015 at 03:52:54PM -0400, Chris Metcalf wrote:
> 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.

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.
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