Re: [PATCH v3 2/2] irqchip/gic-v3-its: Balance initial LPI affinity across CPUs

[John Garry]

> > > +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?

It seems to be just irqbalance setting the affinity mask via sysfs:

[44.782116] Calltrace:
[44.782119] its_select_cpu+0x420/0x6e0
[44.782121] its_set_affinity+0x180/0x208
[44.782126] msi_domain_set_affinity+0x44/0xb8
[44.782130] irq_do_set_affinity+0x48/0x190
[44.782132] irq_set_affinity_locked+0xc0/0xe8
[44.782134] __irq_set_affinity+0x48/0x78
[44.782136] write_irq_affinity.isra.8+0xec/0x110
[44.782138] irq_affinity_proc_write+0x1c/0x28
[44.782142] proc_reg_write+0x70/0xb8
[44.782147] __vfs_write+0x18/0x40
[44.782149] vfs_write+0xb0/0x1d0
[44.782151] ksys_write+0x64/0xe8
[44.782154] __arm64_sys_write+0x18/0x20
[44.782157] el0_svc_common.constprop.2+0x88/0x150
[44.782159] do_el0_svc+0x20/0x80
[44.782162] el0_sync_handler+0x118/0x188
[44.782164] el0_sync+0x140/0x180

And for some reason fancied cpu62.

> > > +			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.

No, this one (D06ES) has a single ITS:

john@ubuntu:~/kernel-dev$ dmesg | grep ITS
[    0.000000] SRAT: PXM 0 -> ITS 0 -> Node 0
[    0.000000] ITS [mem 0x202100000-0x20211ffff]
[    0.000000] ITS@0x0000000202100000: Using ITS number 0
[    0.000000] ITS@0x0000000202100000: allocated 8192 Devices 
@23ea9f0000 (indirect, esz 8, psz 16K, shr 1)
[    0.000000] ITS@0x0000000202100000: allocated 2048 Virtual CPUs 
@23ea9d8000 (indirect, esz 16, psz 4K, shr 1)
[    0.000000] ITS@0x0000000202100000: allocated 256 Interrupt 
Collections @23ea9d3000 (flat, esz 16, psz 4K, shr 1)
[    0.000000] ITS: Using DirectLPI for VPE invalidation
[    0.000000] ITS: Enabling GICv4 support
[    0.044034] Platform MSI: ITS@0x202100000 domain created
[    0.044042] PCI/MSI: ITS@0x202100000 domain created

D06CS has 2x ITS, as you may know :)

And, FWIW, the device is on the 2nd socket, numa node #2.

So the cpu mask of node #0 (where the ITS lives) is 0-23. So no 
intersection with what userspace requested.

> > 	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.

We seem to be following what Thomas described for a non-managed 
interrupt bound to a node. But is this interrupt bound to the node?

Regardless of that, what you're saying seems right - keep local 
interrupt bound to the node. But the problem is that userspace is doing 
its own thing.

> What I wonder is how we end-up with this silly aff_mask the first place.

Cheers,
John

BTW, sorry if any text formatting is mangled. I have to improve my WFH 
setup....