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

From: Marc Zyngier <[email-masked]>
Date: Wed, 18 Mar 2020 17:30:44 +0000

On 2020-03-18 15:34, John Garry wrote:
>>>> +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.

Hmmm. OK. I'm surprised that irqbalance dries to set a range of CPUs, 
instead of
a particular CPU though.

> 
>> 
>>> 
>>>> +			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

There's something I'm missing here. If there's a single ITS in the 
system,
node affinity must cover the whole system, not half of it.

> D06CS has 2x ITS, as you may know :)
> 
> And, FWIW, the device is on the 2nd socket, numa node #2.

You've lost me. Single ITS, but two sockets?

> 
> 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?

If the ITS advertizes affinity to a node (through SRAT, for example),
we should use that. And that's what we have in this patch.

> 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.

Unless you tell the interrupt subsystem that userspace cannot balance
this interrupt, it can happen.

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

You're doing fine! ;-)

         M.
-- 
Jazz is not dead. It just smells funny...

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