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

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Message-ID: <52C33581.1030204@redhat.com>
Date:	Tue, 31 Dec 2013 16:22:09 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	rui wang <ruiv.wang@...il.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 12/30/2013 09:58 PM, rui wang wrote:
> On 12/30/13, Prarit Bhargava <prarit@...hat.com> wrote:
>>
>>
>> On 12/30/2013 07:56 AM, rui wang wrote:
>>
> ...
>> Okay, so the big issue is that we need to do the calculation without this cpu,
> 
>>
>> 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?

Okay, how about,
                        if (irq_has_action(irq) && !irqd_is_per_cpu(data) &&
                            ((!cpumask_empty(&affinity_new)) &&
                              !cpumask_subset(&affinity_new, &online_new)) ||
                             cpumask_empty(&affinity_new))
                                this_count++;

I tried this with the following examples and AFAICT I get the correct result:

1) affinity mask = online mask = 0xf.  CPU 3 (1000b) is down'd.

this_count is not incremented.

2) affinity mask is a non-zero subset of the online mask (which IMO is
the "typical" case).  For example, affinity_mask = 0x9, online mask = 0xf.  CPU
3 is again down'd.

this_count is not incremented.

3) affinity_mask = 0x1, online mask = 0x3. (this is your example).  CPU
1 is going down.

this_count is incremented, as the resulting affinity mask will be 0.

4) affinity_mask = 0x0, online mask = 0x7.  CPU 1 is going down.

this_count is incremented, as the affinity mask is 0.

P.
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