Message-ID: <7bf746da-a84e-5e1b-67bb-bb223893435b@redhat.com>
Date:   Tue, 31 Jul 2018 09:41:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        David Miller <davem@...emloft.net>
Cc:     caleb.raitto@...il.com, mst@...hat.com, netdev@...r.kernel.org,
        caraitto@...gle.com
Subject: Re: [PATCH net-next] virtio_net: force_napi_tx module param.



On 2018年07月31日 03:51, Stephen Hemminger wrote:
> On Sun, 29 Jul 2018 09:00:27 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
>
>> From: Caleb Raitto <caleb.raitto@...il.com>
>> Date: Mon, 23 Jul 2018 16:11:19 -0700
>>
>>> From: Caleb Raitto <caraitto@...gle.com>
>>>
>>> The driver disables tx napi if it's not certain that completions will
>>> be processed affine with tx service.
>>>
>>> Its heuristic doesn't account for some scenarios where it is, such as
>>> when the queue pair count matches the core but not hyperthread count.
>>>
>>> Allow userspace to override the heuristic. This is an alternative
>>> solution to that in the linked patch. That added more logic in the
>>> kernel for these cases, but the agreement was that this was better left
>>> to user control.
>>>
>>> Do not expand the existing napi_tx variable to a ternary value,
>>> because doing so can break user applications that expect
>>> boolean ('Y'/'N') instead of integer output. Add a new param instead.
>>>
>>> Link: https://patchwork.ozlabs.org/patch/725249/
>>> Acked-by: Willem de Bruijn <willemb@...gle.com>
>>> Acked-by: Jon Olson <jonolson@...gle.com>
>>> Signed-off-by: Caleb Raitto <caraitto@...gle.com>
>> So I looked into the history surrounding these issues.
>>
>> First of all, it's always ends up turning out crummy when drivers start
>> to set affinities themselves.  The worst possible case is to do it
>> _conditionally_, and that is exactly what virtio_net is doing.
>>
>>  From the user's perspective, this provides a really bad experience.
>>
>> So if I have a 32-queue device and there are 32 cpus, you'll do all
>> the affinity settings, stopping Irqbalanced from doing anything
>> right?
>>
>> So if I add one more cpu, you'll say "oops, no idea what to do in
>> this situation" and not touch the affinities at all?
>>
>> That makes no sense at all.
>>
>> If the driver is going to set affinities at all, OWN that decision
>> and set it all the time to something reasonable.
>>
>> Or accept that you shouldn't be touching this stuff in the first place
>> and leave the affinities alone.
>>
>> Right now we're kinda in a situation where the driver has been setting
>> affinities in the ncpus==nqueues cases for some time, so we can't stop
>> doing it.
>>
>> Which means we have to set them in all cases to make the user
>> experience sane again.
>>
>> I looked at the linked to patch again:
>>
>> 	https://patchwork.ozlabs.org/patch/725249/
>>
>> And I think the strategy should be made more generic, to get rid of
>> the hyperthreading assumptions.  I also agree that the "assign
>> to first N cpus" logic doesn't make much sense either.
>>
>> Just distribute across the available cpus evenly, and be done with it.
>> If you have 64 cpus and 32 queues, this assigns queues to every other
>> cpu.
>>
>> Then we don't need this weird new module parameter.
> I wonder if it would be possible to give irqbalanced hints
> with irq_set_affinity_hint instead of doing direct affinity setting?

We do use hints instead of affinity directly.

See vp_set_vq_affinity(), which did:

     if (vp_dev->msix_enabled) {
         mask = vp_dev->msix_affinity_masks[info->msix_vector];
         irq = pci_irq_vector(vp_dev->pci_dev, info->msix_vector);
         if (cpu == -1)
             irq_set_affinity_hint(irq, NULL);
         else {
             cpumask_clear(mask);
             cpumask_set_cpu(cpu, mask);
             irq_set_affinity_hint(irq, mask);
         }
     }

Thanks.

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