linux-kernel - Re: question about isolcpus and nohz

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Date:   Fri, 10 Sep 2021 06:14:43 +0200
From:   Mike Galbraith <efault@....de>
To:     Chris Friesen <chris.friesen@...driver.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: question about isolcpus and nohz_full

On Thu, 2021-09-09 at 10:26 -0600, Chris Friesen wrote:
>
> 2) Is it allowed to specify  "nohz_full" for some CPUs at boot time
> without specifying any isolcpus?

Yup (IM[not the least bit;]HO the proper way to partition a box).

>   If so, what happens if I later isolate
> a subset of those CPUs using "cpuset.sched_load_balance" in cgroups?  Is
> that allowed when the equivalent boot args are not?

That's what an old shield script I still have laying around does.  I
booted master on my little desktop box with nohz_full=1,2,3,5,6,7 and
shielded cores 2 and 3, after taking down cpus 4-7 (smt), and it still
seems to work fine.

I used to also override (via ugly.. maybe even fugly, hack) nohz
dynamically, turning the tick on/off for subsets, on having proven best
for jitter of heftily threaded RT app spread across many isolated
cores, thus could at need even partition a box with a mixture of
ticked, nohz idle, and tickless sets, albeit in a rather limited
fashion due to nohz_full preallocation requirement.  Would be nice for
some situations if nohz mode were to become a fully dynamic set
attribute.

	-Mike