linux-kernel - Re: [PATCH 1/3] x86/resctrl: Move setting task's active CPU in a mask into helpers

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Date:   Mon, 7 Dec 2020 19:29:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, tony.luck@...el.com,
        kuo-lang.tseng@...el.com, shakeelb@...gle.com,
        valentin.schneider@....com, mingo@...hat.com, babu.moger@....com,
        james.morse@....com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/resctrl: Move setting task's active CPU in a
 mask into helpers

On Thu, Dec 03, 2020 at 03:25:48PM -0800, Reinette Chatre wrote:
> From: Fenghua Yu <fenghua.yu@...el.com>
> 
> The code of setting the CPU on which a task is running in a CPU mask is
> moved into a couple of helpers.

Pls read section "2) Describe your changes" in
Documentation/process/submitting-patches.rst for more details.

More specifically:

"Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour."

> The new helper task_on_cpu() will be reused shortly.

"reused shortly"? I don't think so.

> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Cc: stable@...r.kernel.org

Fixes?

I guess the same commit from the other two:

Fixes: e02737d5b826 ("x86/intel_rdt: Add tasks files")

?

> ---
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 47 +++++++++++++++++++-------
>  1 file changed, 34 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 6f4ca4bea625..68db7d2dec8f 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -525,6 +525,38 @@ static void rdtgroup_remove(struct rdtgroup *rdtgrp)
>  	kfree(rdtgrp);
>  }
>  
> +#ifdef CONFIG_SMP
> +/* Get the CPU if the task is on it. */
> +static bool task_on_cpu(struct task_struct *t, int *cpu)
> +{
> +	/*
> +	 * This is safe on x86 w/o barriers as the ordering of writing to
> +	 * task_cpu() and t->on_cpu is reverse to the reading here. The
> +	 * detection is inaccurate as tasks might move or schedule before
> +	 * the smp function call takes place. In such a case the function
> +	 * call is pointless, but there is no other side effect.
> +	 */
> +	if (t->on_cpu) {
> +		*cpu = task_cpu(t);

Why have an I/O parameter when you can make it simply:

static int task_on_cpu(struct task_struct *t)
{
	if (t->on_cpu)
		return task_cpu(t);

	return -1;
}

> +
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
> +static void set_task_cpumask(struct task_struct *t, struct cpumask *mask)
> +{
> +	int cpu;
> +
> +	if (mask && task_on_cpu(t, &cpu))
> +		cpumask_set_cpu(cpu, mask);

And that you can turn into:

	if (!mask)
		return;

	cpu = task_on_cpu(t);
	if (cpu < 0)
		return;

	cpumask_set_cpu(cpu, mask);

Readable and simple.

Hmm?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette