On Sun, Jul 29, 2018 at 4:48 PM Michael S. Tsirkin <m...@redhat.com> wrote: > > On Sun, Jul 29, 2018 at 09:00:27AM -0700, David Miller wrote: > > From: Caleb Raitto <caleb.rai...@gmail.com> > > Date: Mon, 23 Jul 2018 16:11:19 -0700 > > > > > From: Caleb Raitto <carai...@google.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 <will...@google.com> > > > Acked-by: Jon Olson <jonol...@google.com> > > > Signed-off-by: Caleb Raitto <carai...@google.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. > > Can't we set affinity to a set of CPUs? > > The point really is that tx irq handler needs a lock on the tx queue to > free up skbs, so processing it on another CPU while tx is active causes > cache line bounces. So we want affinity to CPUs that submit to this > queue on the theory they have these cache line(s) anyway. > > I suspect it's not a uniqueue property of virtio.
It is a good heuristic. But as Jon pointed out, there is a trade-off with other costs, such as increased interrupt load with additional tx queues. This is particularly stark when multiple hyperthreads share a cache, but have independent tx queues.