From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net-next] virtio_net: force_napi_tx module param. Date: Tue, 31 Jul 2018 15:34:08 +0300 Message-ID: <20180731153204-mutt-send-email-mst@kernel.org> References: <20180723231119.142904-1-caleb.raitto@gmail.com> <20180729.090027.1373538625446665385.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: David Miller , caleb.raitto@gmail.com, Jason Wang , Network Development , Caleb Raitto To: Willem de Bruijn Return-path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:43634 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1732093AbeGaOOU (ORCPT ); Tue, 31 Jul 2018 10:14:20 -0400 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On Sun, Jul 29, 2018 at 05:32:56PM -0400, Willem de Bruijn wrote: > On Sun, Jul 29, 2018 at 12:01 PM David Miller wrote: > > > > From: Caleb Raitto > > Date: Mon, 23 Jul 2018 16:11:19 -0700 > > > > > From: Caleb Raitto > > > > > > 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 > > > Acked-by: Jon Olson > > > Signed-off-by: Caleb Raitto > > > > 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. > > Sounds good to me. So e.g. we could set an affinity hint to a group of CPUs that might transmit to this queue. > > If you have 64 cpus and 32 queues, this assigns queues to every other > > cpu. > > Striping half the number of queues as cores on a hyperthreaded system > with two logical cores per physical core will allocate all queues on > only half the physical cores. > > But it is probably not safe to make any assumptions on virtual to > physical core mapping, anyway, which makes the simplest strategy is > preferable.