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

[Dawei Li]

Hi Marc,

Thanks for the review.

On Fri, Apr 12, 2024 at 02:53:32PM +0100, Marc Zyngier wrote:
> On Fri, 12 Apr 2024 11:58:36 +0100,
> Dawei Li <dawei.li@xxxxxxxxxxxx> 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.

Sorry for the confusion.

I didn't mean current implementation is 'improper', actually both
implementations share equivalent API usages. I will remove this
misleading expression from commit message.

> 
> >
> > Signed-off-by: Dawei Li <dawei.li@xxxxxxxxxxxx>
> > ---
> >  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.

Actually maybe it's possible to merge these 2 bitmap ops into one:

diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index fca888b36680..7a267777bd0b 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -3826,7 +3826,8 @@ 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 int common;
        unsigned long flags;
        int from, cpu;

@@ -3850,10 +3851,13 @@ 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);
-       else
+       if (table_mask && (common = cpumask_first_and(mask_val, table_mask)) < nr_cpu_ids) {
+               cpu = cpumask_test_cpu(from, mask_val) &&
+                     cpumask_test_cpu(from, table_mask) ?
+                     from : common;
+       } else {
                cpu = cpumask_first(mask_val);
+       }

> 
> 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.

With change above, I assume that the tradeoff is minor and can be ignored?

And I aplogize if I am missing something.

> 
> > +	}
> >  	else
> >  		cpu = cpumask_first(mask_val);
> 
> Please fix the coding style (if () { ... } else { ... }).

Ack.


Thanks,

    Dawei

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