Messages in this thread | | | Date | Wed, 18 Mar 2020 14:04:06 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH v3 2/2] irqchip/gic-v3-its: Balance initial LPI affinity across CPUs |
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On 2020-03-18 12:22, John Garry wrote: > I may have an idea about this: > irq 196, cpu list 0-31, effective list 82 > > Just going back to comment on the code: > >> +/* >> + * As suggested by Thomas Gleixner in: >> + * https://lore.kernel.org/r/87h80q2aoc.fsf@nanos.tec.linutronix.de >> + */ >> +static int its_select_cpu(struct irq_data *d, >> + const struct cpumask *aff_mask) >> +{ >> + struct its_device *its_dev = irq_data_get_irq_chip_data(d); >> + cpumask_var_t tmpmask; >> + int cpu, node; >> + >> + if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL)) >> + return -ENOMEM; >> + >> + node = its_dev->its->numa_node; >> + >> + if (!irqd_affinity_is_managed(d)) { >> + /* First try the NUMA node */ >> + if (node != NUMA_NO_NODE) { >> + /* >> + * Try the intersection of the affinity mask and the >> + * node mask (and the online mask, just to be safe). >> + */ >> + cpumask_and(tmpmask, cpumask_of_node(node), aff_mask); >> + cpumask_and(tmpmask, tmpmask, cpu_online_mask); >> + >> + /* If that doesn't work, try the nodemask itself */ > > So if tmpmsk is empty...
Which means the proposed affinity mask isn't part of the node mask the first place. Why did we get such an affinity the first place?
> >> + if (cpumask_empty(tmpmask)) >> + cpumask_and(tmpmask, cpumask_of_node(node), cpu_online_mask); > > now the tmpmask may have no intersection with the aff_mask...
But it has the mask for CPUs that are best suited for this interrupt, right? If I understand the topology of your machine, it has an ITS per 64 CPUs, and this device is connected to the ITS that serves the second socket.
> >> + >> + cpu = cpumask_pick_least_loaded(d, tmpmask); >> + if (cpu < nr_cpu_ids) >> + goto out; >> + >> + /* If we can't cross sockets, give up */ >> + if ((its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144)) >> + goto out; >> + >> + /* If the above failed, expand the search */ >> + } > > SNIP > >> +out: >> + free_cpumask_var(tmpmask); >> + >> + pr_debug("IRQ%d -> %*pbl CPU%d\n", d->irq, >> cpumask_pr_args(aff_mask), cpu); >> + return cpu; >> +} >> + >> static int its_set_affinity(struct irq_data *d, const struct cpumask >> *mask_val, >> bool force) >> { >> - unsigned int cpu; >> - const struct cpumask *cpu_mask = cpu_online_mask; >> struct its_device *its_dev = irq_data_get_irq_chip_data(d); >> struct its_collection *target_col; >> u32 id = its_get_event_id(d); >> + int cpu; >> /* A forwarded interrupt should use irq_set_vcpu_affinity */ >> if (irqd_is_forwarded_to_vcpu(d)) >> return -EINVAL; >> - /* lpi cannot be routed to a redistributor that is on a >> foreign node */ >> - if (its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144) { >> - if (its_dev->its->numa_node >= 0) { >> - cpu_mask = cpumask_of_node(its_dev->its->numa_node); >> - if (!cpumask_intersects(mask_val, cpu_mask)) >> - return -EINVAL; >> - } >> - } >> - >> - cpu = cpumask_any_and(mask_val, cpu_mask); >> + if (!force) >> + cpu = its_select_cpu(d, mask_val); >> + else >> + cpu = cpumask_pick_least_loaded(d, mask_val); >> - if (cpu >= nr_cpu_ids) >> + if (cpu < 0 || cpu >= nr_cpu_ids) >> return -EINVAL; > > Annotate missing code: > > if (cpu < 0 || cpu >= nr_cpu_ids) > return -EINVAL; > > if (cpu != its_dev->event_map.col_map[id]) { > its_inc_lpi_count(d, cpu); > its_dec_lpi_count(d, its_dev->event_map.col_map[id]); > target_col = &its_dev->its->collections[cpu]; > its_send_movi(its_dev, target_col, id); > its_dev->event_map.col_map[id] = cpu; > irq_data_update_effective_affinity(d, cpumask_of(cpu)); > } > > So cpu may not be a member of mask_val. Hence the inconsistency of the > affinity list and effective affinity. We could just drop the AND of > the ITS node mask in its_select_cpu().
That would be a departure from the algorithm Thomas proposed, which made a lot of sense in my opinion. What its_select_cpu() does in this case is probably the best that can be achieved from a latency perspective, as it keeps the interrupt local to the socket that generated it.
What I wonder is how we end-up with this silly aff_mask the first place.
> Anyway, I don't think that this should stop us testing.
Agreed.
M. -- Jazz is not dead. It just smells funny...
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