netdev - Re: [PATCH net-next] virtio_net: force_napi

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Date:   Mon, 30 Jul 2018 12:51:46 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Miller <davem@...emloft.net>
Cc:     caleb.raitto@...il.com, mst@...hat.com, jasowang@...hat.com,
        netdev@...r.kernel.org, caraitto@...gle.com
Subject: Re: [PATCH net-next] virtio_net: force_napi_tx module param.

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?