linux-kernel - Re: [PATCH] x86/irq: Do not check available vectors if current CPU has no irq to migrate

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Message-ID: <58EB6BE7.6020802@redhat.com>
Date:   Mon, 10 Apr 2017 07:26:31 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Chen Yu <yu.c.chen@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <len.brown@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/irq: Do not check available vectors if current CPU
 has no irq to migrate



On 04/09/2017 09:53 AM, Chen Yu wrote:
> This is an optimization to bypass the free vector checking if the current
> CPU has no irq to migrate. This can especially speed up the CPU offline
> process when there are many CPUs in the system.

Chen, OOC how much time does this save?

P.

> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> Cc: Len Brown <len.brown@...el.com>
> Cc: x86@...nel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>  arch/x86/kernel/irq.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> index 4d8183b..b9bdc1e 100644
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -394,6 +394,9 @@ int check_irq_vectors_for_cpu_disable(void)
>  		    !cpumask_subset(&affinity_new, &online_new))
>  			this_count++;
>  	}
> +	/* No need to check any further. */
> +	if (!this_count)
> +		return 0;
>  
>  	count = 0;
>  	for_each_online_cpu(cpu) {
>