Michael S. Tsirkin
2021-Apr-13 05:15 UTC
[PATCH net] virtio-net: suppress bad irq warning for tx napi
On Fri, Feb 05, 2021 at 02:28:33PM -0800, Wei Wang wrote:> On Thu, Feb 4, 2021 at 12:48 PM Willem de Bruijn > <willemdebruijn.kernel at gmail.com> wrote: > > > > On Wed, Feb 3, 2021 at 6:53 PM Wei Wang <weiwan at google.com> wrote: > > > > > > On Wed, Feb 3, 2021 at 3:10 PM Michael S. Tsirkin <mst at redhat.com> wrote: > > > > > > > > On Wed, Feb 03, 2021 at 01:24:08PM -0500, Willem de Bruijn wrote: > > > > > On Wed, Feb 3, 2021 at 5:42 AM Michael S. Tsirkin <mst at redhat.com> wrote: > > > > > > > > > > > > On Tue, Feb 02, 2021 at 07:06:53PM -0500, Willem de Bruijn wrote: > > > > > > > On Tue, Feb 2, 2021 at 6:53 PM Willem de Bruijn <willemb at google.com> wrote: > > > > > > > > > > > > > > > > On Tue, Feb 2, 2021 at 6:47 PM Wei Wang <weiwan at google.com> wrote: > > > > > > > > > > > > > > > > > > On Tue, Feb 2, 2021 at 3:12 PM Michael S. Tsirkin <mst at redhat.com> wrote: > > > > > > > > > > > > > > > > > > > > On Thu, Jan 28, 2021 at 04:21:36PM -0800, Wei Wang wrote: > > > > > > > > > > > With the implementation of napi-tx in virtio driver, we clean tx > > > > > > > > > > > descriptors from rx napi handler, for the purpose of reducing tx > > > > > > > > > > > complete interrupts. But this could introduce a race where tx complete > > > > > > > > > > > interrupt has been raised, but the handler found there is no work to do > > > > > > > > > > > because we have done the work in the previous rx interrupt handler. > > > > > > > > > > > This could lead to the following warning msg: > > > > > > > > > > > [ 3588.010778] irq 38: nobody cared (try booting with the > > > > > > > > > > > "irqpoll" option) > > > > > > > > > > > [ 3588.017938] CPU: 4 PID: 0 Comm: swapper/4 Not tainted > > > > > > > > > > > 5.3.0-19-generic #20~18.04.2-Ubuntu > > > > > > > > > > > [ 3588.017940] Call Trace: > > > > > > > > > > > [ 3588.017942] <IRQ> > > > > > > > > > > > [ 3588.017951] dump_stack+0x63/0x85 > > > > > > > > > > > [ 3588.017953] __report_bad_irq+0x35/0xc0 > > > > > > > > > > > [ 3588.017955] note_interrupt+0x24b/0x2a0 > > > > > > > > > > > [ 3588.017956] handle_irq_event_percpu+0x54/0x80 > > > > > > > > > > > [ 3588.017957] handle_irq_event+0x3b/0x60 > > > > > > > > > > > [ 3588.017958] handle_edge_irq+0x83/0x1a0 > > > > > > > > > > > [ 3588.017961] handle_irq+0x20/0x30 > > > > > > > > > > > [ 3588.017964] do_IRQ+0x50/0xe0 > > > > > > > > > > > [ 3588.017966] common_interrupt+0xf/0xf > > > > > > > > > > > [ 3588.017966] </IRQ> > > > > > > > > > > > [ 3588.017989] handlers: > > > > > > > > > > > [ 3588.020374] [<000000001b9f1da8>] vring_interrupt > > > > > > > > > > > [ 3588.025099] Disabling IRQ #38 > > > > > > > > > > > > > > > > > > > > > > This patch adds a new param to struct vring_virtqueue, and we set it for > > > > > > > > > > > tx virtqueues if napi-tx is enabled, to suppress the warning in such > > > > > > > > > > > case. > > > > > > > > > > > > > > > > > > > > > > Fixes: 7b0411ef4aa6 ("virtio-net: clean tx descriptors from rx napi") > > > > > > > > > > > Reported-by: Rick Jones <jonesrick at google.com> > > > > > > > > > > > Signed-off-by: Wei Wang <weiwan at google.com> > > > > > > > > > > > Signed-off-by: Willem de Bruijn <willemb at google.com> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > This description does not make sense to me. > > > > > > > > > > > > > > > > > > > > irq X: nobody cared > > > > > > > > > > only triggers after an interrupt is unhandled repeatedly. > > > > > > > > > > > > > > > > > > > > So something causes a storm of useless tx interrupts here. > > > > > > > > > > > > > > > > > > > > Let's find out what it was please. What you are doing is > > > > > > > > > > just preventing linux from complaining. > > > > > > > > > > > > > > > > > > The traffic that causes this warning is a netperf tcp_stream with at > > > > > > > > > least 128 flows between 2 hosts. And the warning gets triggered on the > > > > > > > > > receiving host, which has a lot of rx interrupts firing on all queues, > > > > > > > > > and a few tx interrupts. > > > > > > > > > And I think the scenario is: when the tx interrupt gets fired, it gets > > > > > > > > > coalesced with the rx interrupt. Basically, the rx and tx interrupts > > > > > > > > > get triggered very close to each other, and gets handled in one round > > > > > > > > > of do_IRQ(). And the rx irq handler gets called first, which calls > > > > > > > > > virtnet_poll(). However, virtnet_poll() calls virtnet_poll_cleantx() > > > > > > > > > to try to do the work on the corresponding tx queue as well. That's > > > > > > > > > why when tx interrupt handler gets called, it sees no work to do. > > > > > > > > > And the reason for the rx handler to handle the tx work is here: > > > > > > > > > https://lists.linuxfoundation.org/pipermail/virtualization/2017-April/034740.html > > > > > > > > > > > > > > > > Indeed. It's not a storm necessarily. The warning occurs after one > > > > > > > > hundred such events, since boot, which is a small number compared real > > > > > > > > interrupt load. > > > > > > > > > > > > > > Sorry, this is wrong. It is the other call to __report_bad_irq from > > > > > > > note_interrupt that applies here. > > > > > > > > > > > > > > > Occasionally seeing an interrupt with no work is expected after > > > > > > > > 7b0411ef4aa6 ("virtio-net: clean tx descriptors from rx napi"). As > > > > > > > > long as this rate of events is very low compared to useful interrupts, > > > > > > > > and total interrupt count is greatly reduced vs not having work > > > > > > > > stealing, it is a net win. > > > > > > > > > > > > Right, but if 99900 out of 100000 interrupts were wasted, then it is > > > > > > surely an even greater win to disable interrupts while polling like > > > > > > this. Might be tricky to detect, disabling/enabling aggressively every > > > > > > time even if there's nothing in the queue is sure to cause lots of cache > > > > > > line bounces, and we don't want to enable callbacks if they were not > > > > > > enabled e.g. by start_xmit ... Some kind of counter? > > > > > > > > > > Yes. It was known that the work stealing is more effective in some > > > > > workloads than others. But a 99% spurious rate I had not anticipated. > > > > > > > > > > Most interesting is the number of interrupts suppressed as a result of > > > > > the feature. That is not captured by this statistic. > > > > > > > > > > In any case, we'll take a step back to better understand behavior. And > > > > > especially why this high spurious rate exhibits in this workload with > > > > > many concurrent flows. > > > > > > > > > > > > I've been thinking about it. Imagine work stealing working perfectly. > > > > Each time we xmit a packet, it is stolen and freed. > > > > Since xmit enables callbacks (just in case!) we also > > > > get an interrupt, which is automatically spurious. > > > > > > > > My conclusion is that we shouldn't just work around it but instead > > > > (or additionally?) > > > > reduce the number of interrupts by disabling callbacks e.g. when > > > > a. we are currently stealing packets > > > > or > > > > b. we stole all packets > > > > Agreed. This might prove a significant performance gain at the same time :) > > > > > > > > > Thinking along this line, that probably means, we should disable cb on > > > the tx virtqueue, when scheduling the napi work on the rx side, and > > > reenable it after the rx napi work is done? > > > Also, I wonder if it is too late to disable cb at the point we start > > > to steal pkts or have stolen all pkts. > > > > The earlier the better. I see no benefit to delay until the rx handler > > actually runs. > > > > I've been thinking more on this. I think the fundamental issue here is > that the rx napi handler virtnet_poll() does the tx side work by > calling virtnet_poll_cleantx() without any notification to the tx > side. > I am thinking, in virtnet_poll(), instead of directly call > virtnet_poll_cleantx(), why not do virtqueue_napi_schedule() to > schedule the tx side napi, and let the tx napi handler do the cleaning > work. This way, we automatically call virtqueue_disable_cb() on the tx > vq, and after the tx work is done, virtqueue_napi_complete() is called > to re-enable the cb on the tx side. This way, the tx side knows what > has been done, and will likely reduce the # of spurious tx interrupts? > And I don't think there is much cost in doing that, since > napi_schedule() basically queues the tx napi to the back of its > napi_list, and serves it right after the rx napi handler is done. > What do you guys think? I could quickly test it up to see if it solves > the issue.Sure pls test. I think you will want to disable event index for now to make sure disable cb is not a nop (I am working on fixing that).> > > Because the steal work is done > > > in the napi handler of the rx queue. But the tx interrupt must have > > > been raised before that. Will we come back to process the tx interrupt > > > again after we re-enabled the cb on the tx side? > > > > > > > This should be enough to reduce the chances below 99% ;) > > > > > > > > One annoying thing is that with split and event index, we do not disable > > > > interrupts. Could be worth revisiting, for now maybe just disable the > > > > event index feature? I am not sure it is actually worth it with > > > > stealing. > > > > With event index, we suppress interrupts when another interrupt is > > already pending from a previous packet, right? When the previous > > position of the producer is already beyond the consumer. It doesn't > > matter whether the previous packet triggered a tx interrupt or > > deferred to an already scheduled rx interrupt? From that seems fine to > > leave it out.