Messages in this thread Patch in this message | | | Date | Sat, 13 Apr 2024 12:11:20 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH 3/6] irqchip/gic-v3-its: Avoid explicit cpumask allocation on stack |
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On Sat, 13 Apr 2024 11:29:20 +0100, Dawei Li <dawei.li@shingroup.cn> wrote: > > 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@shingroup.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. > > 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@shingroup.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. > > 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?
Yup, this works. My preference would be something which I find slightly more readable though (avoiding assignment in the conditional):
diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c index fca888b36680..299dafc7c0ea 100644 --- a/drivers/irqchip/irq-gic-v3-its.c +++ b/drivers/irqchip/irq-gic-v3-its.c @@ -3826,9 +3826,9 @@ 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; + int from, cpu = nr_cpu_ids; /* * Changing affinity is mega expensive, so let's be as lazy as @@ -3850,10 +3850,15 @@ 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) + cpu = cpumask_any_and(mask_val, table_mask); + if (cpu < nr_cpu_ids) { + if (cpumask_test_cpu(from, mask_val) && + cpumask_test_cpu(from, table_mask)) + cpu = from; + } else { cpu = cpumask_first(mask_val); + } if (from == cpu) goto out; Thanks,
M.
-- Without deviation from the norm, progress is not possible.
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