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

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Message-ID: <861pjjcqdi.wl-maz@kernel.org>
Date: Wed, 21 Jan 2026 08:38:17 +0000
From: Marc Zyngier <maz@...nel.org>
To: Waiman Long <longman@...hat.com>,
	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 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.