question about isolcpus and nohz_full

From: Chris Friesen
Date: Thu Sep 09 2021 - 12:26:13 EST


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?