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

From: Chris Metcalf
Date: Mon Apr 13 2015 - 11:48:40 EST

On 04/10/2015 05:11 PM, Andrew Morton wrote:
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?

Thanks for the feedback. It's easy to assume everyone knows
everything about what's being done in the kernel :-)

I'll add some more descriptive language around what the point
of nohz_full is, and why the watchdog interferes with it, in v8.

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?

Chris Metcalf, EZChip Semiconductor

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