linux-kernel - Re: [PATCH 1/2] torture: use for_each_present() loop in torture_online

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Message-ID: <yt9dr0y29jp3.fsf@linux.ibm.com>
Date:   Thu, 17 Nov 2022 07:30:32 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH 1/2] torture: use for_each_present() loop in
 torture_online_all()

Hi Paul,

"Paul E. McKenney" <paulmck@...nel.org> writes:

>> > Yes, rcutorture has lower-level checks for CPUs being hotplugged
>> > behind its back.  Which might be sufficient.  But this patch is in
>> > response to something bad happening if the CPU is also not present in
>> > the cpu_present_mask.  Would that same bad thing happen if rcutorture saw
>> > the CPU in cpu_online_mask, but by the time it attempted to CPU-hotplug
>> > it, that CPU was gone not just from cpu_online_mask, but also from
>> > cpu_present_mask?
>> >
>> > Or are CPUs never removed from cpu_present_mask?
>> 
>> In the current implementation CPUs can only be added to the
>> cpu_present_mask, but never removed. This might change in the future
>> when we get support from firmware for that, but the current s390 code
>> doesn't do that.
>
> Very good!
>
> Then could the patch please check that bits are never removed?
> That way the code will complain should firmware support be added.
>
> 							Thanx, Paul

I'm not sure whether i fully understand that. If the CPU could
be removed from the system and the cpu_present_mask, that could
happen at any time. So i don't see how we should check about that?

Regards
Sven