Re: [tip:sched/core] sched/isolation: Require a present CPU in housekeeping mask

[Frederic Weisbecker]

On Fri, May 03, 2019 at 10:47:37AM -0700, tip-bot for Nicholas Piggin wrote:
> Commit-ID:  9219565aa89033a9cfdae788c1940473a1253d6c
> Gitweb:     https://git.kernel.org/tip/9219565aa89033a9cfdae788c1940473a1253d6c
> Author:     Nicholas Piggin <npiggin@xxxxxxxxx>
> AuthorDate: Thu, 11 Apr 2019 13:34:47 +1000
> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> CommitDate: Fri, 3 May 2019 19:42:58 +0200
> 
> sched/isolation: Require a present CPU in housekeeping mask
> 
> During housekeeping mask setup, currently a possible CPU is required.
> That does not guarantee the CPU would be available at boot time, so
> check to ensure that at least one present CPU is in the mask.

I have a doubt about the requirements and semantics of cpu_present_mask.
IIUC a present CPU means that it is physically plugged in (from ACPI
perspective) but might not be logically plugged in (set on cpu_online_mask).

But do we have the guarantee that a present CPU _will_ be online at least once
right after the boot? After all, kernel parameters such as "maxcpus=" can prevent
from turning some CPUs on. I guess there are even more creative ways to achieve
that.

In any case we really require the housekeeper to be forced online. Perhaps
I missed that enforcement somewhere in the patchset?

Thanks.