linux-kernel - Re: [PATCH v6 00/29] context_tracking,x86: Defer some IPIs until a user->kernel transition

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Message-ID: <xhsmh5xbn5vid.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Thu, 06 Nov 2025 11:02:34 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v6 00/29] context_tracking,x86: Defer some IPIs until a
 user->kernel transition

On 05/11/25 18:46, Frederic Weisbecker wrote:
> Le Wed, Nov 05, 2025 at 05:24:29PM +0100, Valentin Schneider a écrit :
>> On 29/10/25 18:15, Frederic Weisbecker wrote:
>> > Le Wed, Oct 29, 2025 at 11:32:58AM +0100, Valentin Schneider a écrit :
>> >> I need to have a think about that one; one pain point I see is the context
>> >> tracking work has to be NMI safe since e.g. an NMI can take us out of
>> >> userspace. Another is that NOHZ-full CPUs need to be special cased in the
>> >> stop machine queueing / completion.
>> >>
>> >> /me goes fetch a new notebook
>> >
>> > Something like the below (untested) ?
>> >
>>
>> Some minor nits below but otherwise that looks promising.
>>
>> One problem I'm having however is reasoning about the danger zone; what
>> forbidden actions could a NO_HZ_FULL CPU take when entering the kernel
>> while take_cpu_down() is happening?
>>
>> I'm actually not familiar with why we actually use stop_machine() for CPU
>> hotplug; I see things like CPUHP_AP_SMPCFD_DYING::smpcfd_dying_cpu() or
>> CPUHP_AP_TICK_DYING::tick_cpu_dying() expect other CPUs to be patiently
>> spinning in multi_cpu_stop(), and I *think* nothing in the entry code up to
>> context_tracking entry would disrupt that, but it's not a small thing to
>> reason about.
>>
>> AFAICT we need to reason about every .teardown callback from
>> CPUHP_TEARDOWN_CPU to CPUHP_AP_OFFLINE and their explicit & implicit
>> dependencies on other CPUs being STOP'd.
>
> You're raising a very interesting question. The initial point of stop_machine()
> is to synchronize this:
>
>     set_cpu_online(cpu, 0)
>     migrate timers;
>     migrate hrtimers;
>     flush IPIs;
>     etc...
>
> against this pattern:
>
>     preempt_disable()
>     if (cpu_online(cpu))
>         queue something; // could be timer, IPI, etc...
>     preempt_enable()
>
> There have been attempts:
>
>       https://lore.kernel.org/all/20241218171531.2217275-1-costa.shul@redhat.com/
>
> And really it should be fine to just do:
>
>     set_cpu_online(cpu, 0)
>     synchronize_rcu()
>     migrate / flush stuff
>

That's what I was thinking as well, at the very least for the
cpu_online_mask bit.

> Probably we should try that instead of the busy loop I proposed
> which only papers over the problem.
>
> Of course there are other assumptions. For example the tick
> timekeeper is migrated easily knowing that all online CPUs are
> not idle (cf: tick_cpu_dying()). So I expect a few traps, with RCU
> for example and indeed all these hotplug callbacks must be audited
> one by one.
>
> I'm not entirely unfamiliar with many of them. Let me see what I can do...
>

Here be dragons :-)