From: Thomas Gleixner <tglx@linutronix.de>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-kernel@vger.kernel.org, John Garry <john.garry@huawei.com>,
David Decotigny <ddecotig@google.com>
Subject: Re: [PATCH] genirq/msi: Shutdown managed interrupts with unsatifiable affinities
Date: Mon, 14 Mar 2022 20:03:49 +0100 [thread overview]
Message-ID: <87mthsfjai.ffs@tglx> (raw)
In-Reply-To: <87ilsgzfpv.wl-maz@kernel.org>
On Mon, Mar 14 2022 at 16:00, Marc Zyngier wrote:
> On Mon, 14 Mar 2022 15:27:10 +0000,
> Thomas Gleixner <tglx@linutronix.de> wrote:
>>
>> On Mon, Mar 07 2022 at 19:06, Marc Zyngier wrote:
>> > When booting with maxcpus=<small number>, interrupt controllers
>> > such as the GICv3 ITS may not be able to satisfy the affinity of
>> > some managed interrupts, as some of the HW resources are simply
>> > not available.
>>
>> This is also true if you have offlined lots of CPUs, right?
>
> Not quite. If you offline the CPUs, the interrupts will be placed in
> the shutdown state as expected, having initially transitioned via an
> activation state with an online CPU. The issue here is with the
> initial activation of the interrupt, which currently happens even if
> no matching CPU is present.
Yes. But if you load the driver _after_ offlining lots of CPUs first
then the same thing should happen, right?
>> > + /*
>> > + * If the interrupt is managed but no CPU is available
>> > + * to service it, shut it down until better times.
>> > + */
>> > + if ((vflags & VIRQ_ACTIVATE) &&
>> > + irqd_affinity_is_managed(irqd) &&
>> > + !cpumask_intersects(irq_data_get_affinity_mask(irqd),
>> > + cpu_online_mask)) {
>> > + irqd_set_managed_shutdown(irqd);
>>
>> Hrm. Why is this in the !CAN_RESERVE path and not before the actual
>> activation call?
>
> VIRQ_CAN_RESERVE can only happen as a consequence of
> GENERIC_IRQ_RESERVATION_MODE, which only exists on x86. Given that x86
> is already super careful not to activate an interrupt that is not
> immediately required, I though we could avoid putting this check on
> that path.
>
> But if I got the above wrong (which is, let's face it, extremely
> likely), I'm happy to kick it down the road next to the activation
> call.
I just rechecked. Yes, we could push it there, but actually on x86 the
reservation mode activation sets the entry to a spurious catch all on an
online CPU, which is intentional.
So yes, we can keep it where it is now, but that needs a comment.
Thanks,
tglx
next prev parent reply other threads:[~2022-03-14 19:04 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-07 19:06 [PATCH] genirq/msi: Shutdown managed interrupts with unsatifiable affinities Marc Zyngier
2022-03-08 1:34 ` David Decotigny
2022-03-09 10:20 ` John Garry
2022-03-10 3:24 ` Xiongfeng Wang
2022-03-10 6:11 ` Xiongfeng Wang
2022-03-14 15:27 ` Thomas Gleixner
2022-03-14 16:00 ` Marc Zyngier
2022-03-14 19:03 ` Thomas Gleixner [this message]
2022-03-15 9:46 ` Marc Zyngier
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