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

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Date:   Sat, 04 May 2019 16:59:12 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Frederic Weisbecker <frederic@...nel.org>, fweisbec@...il.com,
        hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...nel.org,
        peterz@...radead.org, rafael.j.wysocki@...el.com,
        tglx@...utronix.de, 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

Frederic Weisbecker's on May 4, 2019 10:27 am:
> 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).

Right, a superset of cpu_possible_mask, subset of cpu_online_mask. It 
means that CPU can be brought online at any time.

> 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?

No I think you're right, that may be able to boot without anything in
the housekeeping mask. Maybe we can just cpu_up() a CPU in the 
housekeeping mask with a warning that it has overidden their SMP
command line option. I'll take a look at it.

Thanks,
Nick