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Message-ID: <87ttjtunbx.ffs@tglx>
Date: Mon, 22 Apr 2024 22:58:10 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Dongli Zhang <dongli.zhang@...cle.com>, linux-kernel@...r.kernel.org
Cc: virtualization@...ts.linux.dev, joe.jin@...cle.com
Subject: Re: [PATCH 1/1] genirq/cpuhotplug: retry with online CPUs on
irq_do_set_affinity failure
On Thu, Apr 18 2024 at 18:33, Dongli Zhang wrote:
> When a CPU is offline, its IRQs may migrate to other CPUs. For managed
> IRQs, they are migrated, or shutdown (if all CPUs of the managed IRQ
> affinity are offline). For regular IRQs, there will only be a
> migration.
Please write out interrupts. There is enough space for it and IRQ is
just not a regular word.
> The migrate_one_irq() first uses pending_mask or affinity_mask of the IRQ.
>
> 104 if (irq_fixup_move_pending(desc, true))
> 105 affinity = irq_desc_get_pending_mask(desc);
> 106 else
> 107 affinity = irq_data_get_affinity_mask(d);
>
> The migrate_one_irq() may use all online CPUs, if all CPUs in
> pending_mask/affinity_mask are already offline.
>
> 113 if (cpumask_any_and(affinity, cpu_online_mask) >= nr_cpu_ids) {
> 114 /*
> 115 * If the interrupt is managed, then shut it down and leave
> 116 * the affinity untouched.
> 117 */
> 118 if (irqd_affinity_is_managed(d)) {
> 119 irqd_set_managed_shutdown(d);
> 120 irq_shutdown_and_deactivate(desc);
> 121 return false;
> 122 }
> 123 affinity = cpu_online_mask;
> 124 brokeaff = true;
> 125 }
Please don't copy code into the change log. Describe the problem in
text.
> However, there is a corner case. Although some CPUs in
> pending_mask/affinity_mask are still online, they are lack of available
> vectors. If the kernel continues calling irq_do_set_affinity() with those CPUs,
> there will be -ENOSPC error.
>
> This is not reasonable as other online CPUs still have many available
> vectors.
Reasonable is not the question here. It's either correct or not.
> name: VECTOR
> size: 0
> mapped: 529
> flags: 0x00000103
> Online bitmaps: 7
> Global available: 884
> Global reserved: 6
> Total allocated: 539
> System: 36: 0-19,21,50,128,236,243-244,246-255
> | CPU | avl | man | mac | act | vectors
> 0 147 0 0 55 32-49,51-87
> 1 147 0 0 55 32-49,51-87
> 2 0 0 0 202 32-49,51-127,129-235
Just ouf of curiousity. How did this end up with CPU2 completely
occupied?
> 4 147 0 0 55 32-49,51-87
> 5 147 0 0 55 32-49,51-87
> 6 148 0 0 54 32-49,51-86
> 7 148 0 0 54 32-49,51-86
>
> This issue should not happen for managed IRQs because the vectors are already
> reserved before CPU hotplug.
Should not? It either does or it does not.
> For regular IRQs, do a re-try with all online
> CPUs if the prior irq_do_set_affinity() is failed with -ENOSPC.
>
> Cc: Joe Jin <joe.jin@...cle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
> ---
> kernel/irq/cpuhotplug.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/kernel/irq/cpuhotplug.c b/kernel/irq/cpuhotplug.c
> index 1ed2b1739363..d1666a6b73f4 100644
> --- a/kernel/irq/cpuhotplug.c
> +++ b/kernel/irq/cpuhotplug.c
> @@ -130,6 +130,19 @@ static bool migrate_one_irq(struct irq_desc *desc)
> * CPU.
> */
> err = irq_do_set_affinity(d, affinity, false);
> +
> + if (err == -ENOSPC &&
> + !irqd_affinity_is_managed(d) &&
> + affinity != cpu_online_mask) {
This really wants to be a single line conditional.
> + affinity = cpu_online_mask;
> + brokeaff = true;
> +
> + pr_debug("IRQ%u: set affinity failed for %*pbl, re-try with all online CPUs\n",
> + d->irq, cpumask_pr_args(affinity));
How is it useful to print cpu_online_mask here?
Thanks,
tglx
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