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Message-ID: <86il0msn4z.wl-maz@kernel.org>
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.
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