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To:     Frederic Weisbecker <frederic@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
From:   Chris Friesen <chris.friesen@windriver.com>
Subject: question about isolcpus and nohz_full
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Date:   Thu, 9 Sep 2021 10:26:04 -0600
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Hi,

I'm finally getting around to moving to a newer kernel, and I'm running 
into a warning "Housekeeping: nohz_full= must match isolcpus=".

In my environment I had a number of "fully-isolated" CPUs with both 
"isolcpus" and "nohz_full", then a number of less-isolated CPUs that had 
"nohz_full" enabled but not "isolcpus", then the housekeeping CPUs.  It 
appears this is no longer supported via the boot args.  (The 
"less-isolated CPUs were used for applications that expected the usual 
load balancing from the scheduler but didn't want to be interrupted by 
timer ticks.)

1) The kernel-parameters documentation for "isolcpus=" doesn't say 
anything about needing to match "nohz_full", nor does the documentation 
for "nohz_full" mention that it needs to align with "isolcpus".  Maybe 
this would be a good thing to add?

2) Is it allowed to specify  "nohz_full" for some CPUs at boot time 
without specifying any isolcpus?  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?

Thanks,

Chris