linux-kernel - Re: [PATCH 3/6] irqchip/gic-v3-its: Avoid explicit cpumask allocation on stack

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Date: Fri, 12 Apr 2024 14:53:32 +0100
From: Marc Zyngier <maz@...nel.org>
To: Dawei Li <dawei.li@...ngroup.cn>
Cc: tglx@...utronix.de,
	yury.norov@...il.com,
	akpm@...ux-foundation.org,
	florian.fainelli@...adcom.com,
	chenhuacai@...nel.org,
	jiaxun.yang@...goat.com,
	anup@...infault.org,
	palmer@...belt.com,
	samuel.holland@...ive.com,
	linux@...musvillemoes.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] irqchip/gic-v3-its: Avoid explicit cpumask allocation on stack

On Fri, 12 Apr 2024 11:58:36 +0100,
Dawei Li <dawei.li@...ngroup.cn> wrote:
> 
> In general it's preferable to avoid placing cpumasks on the stack, as
> for large values of NR_CPUS these can consume significant amounts of
> stack space and make stack overflows more likely.
>
> Remove cpumask var on stack and use proper cpumask API to address it.

Define proper. Or better, define what is "improper" about the current
usage.

>
> Signed-off-by: Dawei Li <dawei.li@...ngroup.cn>
> ---
>  drivers/irqchip/irq-gic-v3-its.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index fca888b36680..a821396c4261 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -3826,7 +3826,7 @@ static int its_vpe_set_affinity(struct irq_data *d,
>  				bool force)
>  {
>  	struct its_vpe *vpe = irq_data_get_irq_chip_data(d);
> -	struct cpumask common, *table_mask;
> +	struct cpumask *table_mask;
>  	unsigned long flags;
>  	int from, cpu;
>  
> @@ -3850,8 +3850,11 @@ static int its_vpe_set_affinity(struct irq_data *d,
>  	 * If we are offered another CPU in the same GICv4.1 ITS
>  	 * affinity, pick this one. Otherwise, any CPU will do.
>  	 */
> -	if (table_mask && cpumask_and(&common, mask_val, table_mask))
> -		cpu = cpumask_test_cpu(from, &common) ? from : cpumask_first(&common);
> +	if (table_mask && cpumask_intersects(mask_val, table_mask)) {
> +		cpu = cpumask_test_cpu(from, mask_val) &&
> +		      cpumask_test_cpu(from, table_mask) ?
> +		      from : cpumask_first_and(mask_val, table_mask);

So we may end-up computing the AND of the two bitmaps twice (once for
cpumask_intersects(), once for cpumask_first_and()), instead of only
doing it once.

I don't expect that to be horrible, but I also note that you don't
even talk about the trade-offs you are choosing to make.

> +	}
>  	else
>  		cpu = cpumask_first(mask_val);

Please fix the coding style (if () { ... } else { ... }).

	M.

-- 
Without deviation from the norm, progress is not possible.