linux-kernel - Re: [PATCH] x86: Add check for number of available vectors before CPU down [v2]

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Date:	Tue, 31 Dec 2013 10:58:13 +0800
From:	rui wang <ruiv.wang@...il.com>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Tony Luck <tony.luck@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86-ML <x86@...nel.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Seiji Aguchi <seiji.aguchi@....com>,
	Yang Zhang <yang.z.zhang@...el.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	janet.morgan@...el.com, "Yu, Fenghua" <fenghua.yu@...el.com>,
	chen gong <gong.chen@...ux.intel.com>
Subject: Re: [PATCH] x86: Add check for number of available vectors before CPU
 down [v2]

On 12/30/13, Prarit Bhargava <prarit@...hat.com> wrote:
>
>
> On 12/30/2013 07:56 AM, rui wang wrote:
>
>> An irq can be mapped to only one vector number, but can have multiple
>> destination CPUs. i.e. the same irq/vector can appear on multiple
>> CPUs' vector_irq[]. So checking data->affinity is necessary I think.
>
> That's true Rui -- but here's what I think the scenario actually is.
>
> Suppose we have a 4-cpu system, and we have an IRQ that is mapped to
> multiple
> cpu's vector_irq[].  For example, we have IRQ 200 that is mapped to CPU 2
> vector_irq[50], and CPU 3 vector_irq[60].
>

This should not happen. There's only one LSB to fill the vector number
in IRTE. So either 50 or 60 but not both for an irq.

...
> Okay, so the big issue is that we need to do the calculation without this cpu,

Yes if it's done before calling cpu_disable_comm().

> so I think this works (sorry for the cut-and-paste)
>
> int check_irq_vectors_for_cpu_disable(void)
> {
>        int irq, cpu;
>        unsigned int vector, this_count, count;
>        struct irq_desc *desc;
>        struct irq_data *data;
>        struct cpumask online_new; /* cpu_online_mask - this_cpu */
>        struct cpumask affinity_new; /* affinity - this_cpu */
>
>        cpumask_copy(&online_new, cpu_online_mask);
>        cpu_clear(smp_processor_id(), online_new);
>
>        this_count = 0;
>        for (vector = FIRST_EXTERNAL_VECTOR; vector < NR_VECTORS; vector++) {
>                irq = __this_cpu_read(vector_irq[vector]);
>                if (irq >= 0) {
>                        desc = irq_to_desc(irq);
>                        data = irq_desc_get_irq_data(desc);
>                        cpumask_copy(&affinity_new, data->affinity);
>                        cpu_clear(smp_processor_id(), affinity_new);
>                        if (irq_has_action(irq) && !irqd_is_per_cpu(data) &&
>                            !cpumask_subset(&affinity_new, &online_new) &&
>                            !cpumask_empty(&affinity_new))

If this cpu is the only target, then affinity_new becomes empty.
Should we count it for migration?

Thanks
Rui
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