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

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Date:   Sat, 4 May 2019 02:27:34 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     hpa@...or.com, tglx@...utronix.de, mingo@...nel.org,
        linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
        npiggin@...il.com, fweisbec@...il.com, peterz@...radead.org,
        torvalds@...ux-foundation.org
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/isolation: Require a present CPU in
 housekeeping mask

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@...il.com>
> AuthorDate: Thu, 11 Apr 2019 13:34:47 +1000
> Committer:  Ingo Molnar <mingo@...nel.org>
> 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.