linux-kernel - Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in allocate_vpe_l1

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Message-ID: <0bada1e3-f312-4b72-974c-4b061e577d66@redhat.com>
Date: Wed, 21 Jan 2026 11:48:41 -0500
From: Waiman Long <llong@...hat.com>
To: Marc Zyngier <maz@...nel.org>, Thomas Gleixner <tglx@...nel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't acquire rt_spin_lock in
 allocate_vpe_l1_table()

On 1/21/26 3:38 AM, Marc Zyngier wrote:
> On Mon, 12 Jan 2026 11:20:07 +0000,
> Marc Zyngier <maz@...nel.org> wrote:
>> On Sun, 11 Jan 2026 16:20:45 +0000,
>> Thomas Gleixner <tglx@...nel.org> wrote:
>>> On Sun, Jan 11 2026 at 10:38, Marc Zyngier wrote:
>>>> On Sun, 11 Jan 2026 09:39:07 +0000,
>>>> Thomas Gleixner <tglx@...nel.org> wrote:
>>>>> On Fri, Jan 09 2026 at 16:13, Marc Zyngier wrote:
>>>>>> On Thu, 08 Jan 2026 22:11:33 +0000,
>>>>>> Thomas Gleixner <tglx@...nel.org> wrote:
>>>>>>> At the point where a CPU is brought up, the topology should be known
>>>>>>> already, which means this can be allocated on the control CPU _before_
>>>>>>> the new CPU comes up, no?
>>>>>> No. Each CPU finds *itself* in the forest of redistributors, and from
>>>>>> there tries to find whether it has some shared resource with a CPU
>>>>>> that has booted before it. That's because firmware is absolutely awful
>>>>>> and can't present a consistent view of the system.
>>>>> Groan....
>>>>>
>>>>>> Anyway, I expect it could be solved by moving this part of the init to
>>>>>> an ONLINE HP callback.
>>>>> Which needs to be before CPUHP_AP_IRQ_AFFINITY_ONLINE, but even that
>>>>> might be to late because there are callbacks in the STARTING section,
>>>>> i.e. timer, perf, which might rely on interrupts being accessible.
>>>> Nah. This stuff is only for direct injection of vLPIs into guests, so
>>>> as long as this is done before we can schedule a vcpu on this physical
>>>> CPU, we're good. No physical interrupt is concerned with this code.
>>> That's fine then. vCPUs are considered "user-space" tasks and can't be
>>> scheduled before CPUHP_AP_ACTIVE sets the CPU active for the scheduler.
>> Waiman, can you please give the following hack a go on your box? The
>> machines I have are thankfully limited to a single ITS group, so I
>> can't directly reproduce your issue.
> Have you managed to try this hack? I may be able to spend some time
> addressing the issue in the next cycle if I have an indication that
> I'm on the right track.

I am sorry that I was busy working on other stuff. Will try out the hack 
today and report back ASAP.

Cheers,
Longman