Messages in this thread | | | Date | Tue, 21 Jun 2022 09:06:17 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH v2 4/6] genirq: Provide an IRQ affinity mask in non-SMP configs |
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On Tue, 21 Jun 2022 05:03:43 +0100, Samuel Holland <samuel@sholland.org> wrote: > > On 6/18/22 4:01 AM, Marc Zyngier wrote: > > Hi Samuel, > > > > On Thu, 16 Jun 2022 07:40:26 +0100, > > Samuel Holland <samuel@sholland.org> wrote: > >> > >> IRQ affinity masks are not allocated in uniprocessor configurations. > >> This requires special case non-SMP code in drivers for irqchips which > >> have per-CPU enable or mask registers. > >> > >> Since IRQ affinity is always the same in a uniprocessor configuration, > >> we can still provide the correct affinity mask without allocating one > >> per IRQ. We can reuse the system-wide cpu_possible_mask. > >> > >> By returning a real cpumask from irq_data_get_affinity_mask even when > >> SMP is disabled, irqchip drivers which iterate over that mask will > >> automatically do the right thing. > >> > >> Signed-off-by: Samuel Holland <samuel@sholland.org> > >> --- > >> > >> (no changes since v1) > >> > >> include/linux/irq.h | 6 ++++++ > >> 1 file changed, 6 insertions(+) > >> > >> diff --git a/include/linux/irq.h b/include/linux/irq.h > >> index 69ee4e2f36ce..d5e958b026aa 100644 > >> --- a/include/linux/irq.h > >> +++ b/include/linux/irq.h > >> @@ -151,7 +151,9 @@ struct irq_common_data { > >> #endif > >> void *handler_data; > >> struct msi_desc *msi_desc; > >> +#ifdef CONFIG_SMP > >> cpumask_var_t affinity; > >> +#endif > >> #ifdef CONFIG_GENERIC_IRQ_EFFECTIVE_AFF_MASK > >> cpumask_var_t effective_affinity; > >> #endif > >> @@ -881,7 +883,11 @@ static inline int irq_data_get_node(struct irq_data *d) > >> > >> static inline struct cpumask *irq_data_get_affinity_mask(struct irq_data *d) > >> { > >> +#ifdef CONFIG_SMP > >> return d->common->affinity; > >> +#else > >> + return &__cpu_possible_mask; > >> +#endif > > > > I have a bad feeling about this one. Being in a !SMP configuration > > doesn't necessarily mean that __cpu_possible_mask only contains a > > single CPU, specially with things like CONFIG_INIT_ALL_POSSIBLE. I can > > also imagine an architecture populating this bitmap from firmware > > tables irrespective of the SMP status of the kernel. > > > > Can't you use something like: > > > > return cpumask_of(0); > > > > which is guaranteed to be the right thing on !SMP configuration? > > I can if I cast away the const. However I see a lot of: > > cpumask_copy(irq_data_get_affinity_mask(d), foo); > > which I suppose is a great reason not to do what I am doing.
Ah, indeed. Not going to work very well...
> The right solution seems to be adding irq_data_update_affinity() to > match irq_data_update_effective_affinity(), and making both getters > return a const cpumask. Then I can use cpumask_of(0).
Sounds like a plan.
Thanks,
M.
-- Without deviation from the norm, progress is not possible.
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