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

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Message-ID: <20221115134139.GI4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 15 Nov 2022 05:41:39 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sven Schnelle <svens@...ux.ibm.com>
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()

On Tue, Nov 15, 2022 at 07:55:50AM +0100, Sven Schnelle wrote:
> Hi Paul,
> 
> "Paul E. McKenney" <paulmck@...nel.org> writes:
> 
> > On Mon, Nov 14, 2022 at 04:35:06PM +0100, Sven Schnelle wrote:
> >> "Paul E. McKenney" <paulmck@...nel.org> writes:
> >> 
> >> > On Fri, Nov 11, 2022 at 01:51:24PM +0100, Sven Schnelle wrote:
> >> >> A CPU listed in the possible mask doesn't have to be present, in
> >> >> which case it would crash the kernel in torture_online_all().
> >> >> To prevent this use a for_each_present() loop.
> >> >> 
> >> >> Signed-off-by: Sven Schnelle <svens@...ux.ibm.com>
> >> >
> >> > Looks good to me!  Any reason for no mailing list on CC?
> >> 
> >> No, my fault. I setup get_maintainer.pl to be called from git
> >> send-email, but looks like i did it wrong :-)
> >
> > Been there, done that!  ;-)
> >
> >> > Ah, and any synchronization required in case it is possible for a CPU
> >> > to leave the cpu_present_mask?  Or can they only be added?
> >> 
> >> Hmm... I think the main question is, whether it is ok for a cpu to be
> >> removed from the system when rcutorture is running? In both cases it
> >> would disappear from the cpu online mask, so i don't think the patch
> >> would change the behaviour. But i can check and send additional patches
> >> if there are other places that needs adjustment.
> >
> > 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