Re: [PATCH v7 1/3] smpboot: allow excluding cpus from the smpboot threads

From: Andrew Morton
Date: Fri Apr 10 2015 - 17:11:47 EST

On Fri, 10 Apr 2015 16:48:18 -0400 Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote:

> This change allows some cores to be excluded from running the
> smp_hotplug_thread tasks. The motivating example for this is
> the watchdog threads, which by default we don't want to run
> on any enabled nohz_full cores.

Why not?

I can guess, but I'd rather not guess. Please fully explain the
end-user value of this change. Providing a benefit to users is the
whole point of the patchset, but the above assertion is the only
description we have.

This info should be in Documentation/lockup-watchdogs.txt and/or
Documentation/sysctl/kernel.txt as well as the changelogs, so users
have an answer to "why the heck should I enable this".

Please also describe the downside of the change. I assume this is
"lockups will go undetected on some CPUs"? Let's expand on this so we
can understand where the best tradeoff point lies.

If people are experiencing <whatever this problem is> then they can
disable the watchdog altogether. What value is there in this partial
disabling? Why is it worth doing this?

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