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[2003:cf:d73b:4143:2bf6:228a:1b7a:e384]) by smtp.gmail.com with ESMTPSA id e30-20020a5d595e000000b00339237a2752sm9866809wri.33.2024.01.23.03.15.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Jan 2024 03:15:50 -0800 (PST) Message-ID: Date: Tue, 23 Jan 2024 12:15:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 0/3] aio-posix: call ->poll_end() when removing AioHandler From: Hanna Czenczek To: Fiona Ebner , Paolo Bonzini , Stefan Hajnoczi , qemu-devel@nongnu.org Cc: Kevin Wolf , qemu-block@nongnu.org, Fam Zheng References: <20231213211544.1601971-1-stefanha@redhat.com> <142d6078-1bb9-4116-ac87-7daac16f12d8@redhat.com> <016ac3d1-f6c1-48eb-a714-fb777dff7012@proxmox.com> <94db88e7-1f02-44dd-bc2c-3d9ccf1cce72@redhat.com> <67a36617-9e61-4778-aebf-1e667cb51120@proxmox.com> <3bb5aa0e-ae0a-4fda-a5b5-1bfac86651ac@redhat.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.289, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 22.01.24 18:52, Hanna Czenczek wrote: > On 22.01.24 18:41, Hanna Czenczek wrote: >> On 05.01.24 15:30, Fiona Ebner wrote: >>> Am 05.01.24 um 14:43 schrieb Fiona Ebner: >>>> Am 03.01.24 um 14:35 schrieb Paolo Bonzini: >>>>> On 1/3/24 12:40, Fiona Ebner wrote: >>>>>> I'm happy to report that I cannot reproduce the CPU-usage-spike >>>>>> issue >>>>>> with the patch, but I did run into an assertion failure when >>>>>> trying to >>>>>> verify that it fixes my original stuck-guest-IO issue. See below >>>>>> for the >>>>>> backtrace [0]. Hanna wrote in >>>>>> https://issues.redhat.com/browse/RHEL-3934 >>>>>> >>>>>>> I think it’s sufficient to simply call virtio_queue_notify_vq(vq) >>>>>>> after the virtio_queue_aio_attach_host_notifier(vq, ctx) call, >>>>>>> because >>>>>>> both virtio-scsi’s and virtio-blk’s .handle_output() >>>>>>> implementations >>>>>>> acquire the device’s context, so this should be directly >>>>>>> callable from >>>>>>> any context. >>>>>> I guess this is not true anymore now that the AioContext locking was >>>>>> removed? >>>>> Good point and, in fact, even before it was much safer to use >>>>> virtio_queue_notify() instead.  Not only does it use the event >>>>> notifier >>>>> handler, but it also calls it in the right thread/AioContext just by >>>>> doing event_notifier_set(). >>>>> >>>> But with virtio_queue_notify() using the event notifier, the >>>> CPU-usage-spike issue is present: >>>> >>>>>> Back to the CPU-usage-spike issue: I experimented around and it >>>>>> doesn't >>>>>> seem to matter whether I notify the virt queue before or after >>>>>> attaching >>>>>> the notifiers. But there's another functional difference. My patch >>>>>> called virtio_queue_notify() which contains this block: >>>>>> >>>>>>>      if (vq->host_notifier_enabled) { >>>>>>> event_notifier_set(&vq->host_notifier); >>>>>>>      } else if (vq->handle_output) { >>>>>>>          vq->handle_output(vdev, vq); >>>>>> In my testing, the first branch was taken, calling >>>>>> event_notifier_set(). >>>>>> Hanna's patch uses virtio_queue_notify_vq() and there, >>>>>> vq->handle_output() will be called. That seems to be the relevant >>>>>> difference regarding the CPU-usage-spike issue. >>>> I should mention that this is with a VirtIO SCSI disk. I also >>>> attempted >>>> to reproduce the CPU-usage-spike issue with a VirtIO block disk, but >>>> didn't manage yet. >>>> >>>> What I noticed is that in virtio_queue_host_notifier_aio_poll(), >>>> one of >>>> the queues (but only one) will always show as nonempty. And then, >>>> run_poll_handlers_once() will always detect progress which explains >>>> the >>>> CPU usage. >>>> >>>> The following shows >>>> 1. vq address >>>> 2. number of times vq was passed to >>>> virtio_queue_host_notifier_aio_poll() >>>> 3. number of times the result of virtio_queue_host_notifier_aio_poll() >>>> was true for the vq >>>> >>>>> 0x555fd93f9c80 17162000 0 >>>>> 0x555fd93f9e48 17162000 6 >>>>> 0x555fd93f9ee0 17162000 0 >>>>> 0x555fd93f9d18 17162000 17162000 >>>>> 0x555fd93f9db0 17162000 0 >>>>> 0x555fd93f9f78 17162000 0 >>>> And for the problematic one, the reason it is seen as nonempty is: >>>> >>>>> 0x555fd93f9d18 shadow_avail_idx 8 last_avail_idx 0 >>> vring_avail_idx(vq) also gives 8 here. This is the vs->event_vq and >>> s->events_dropped is false in my testing, so >>> virtio_scsi_handle_event_vq() doesn't do anything. >>> >>>> Those values stay like this while the call counts above increase. >>>> >>>> So something going wrong with the indices when the event notifier >>>> is set >>>> from QEMU side (in the main thread)? >>>> >>>> The guest is Debian 12 with a 6.1 kernel. >> >> So, trying to figure out a new RFC version: >> >> About the stack trace you, Fiona, posted:  As far as I understand, >> that happens because virtio_blk_drained_end() calling >> virtio_queue_notify_vq() wasn’t safe after all, and instead we need >> to use virtio_queue_notify().  Right? >> >> However, you say using virtio_queue_notify() instead causes busy >> loops of doing nothing in virtio-scsi (what you describe above). I >> mean, better than a crash, but, you know. :) >> >> I don’t know have any prior knowledge about the event handling, >> unfortunately.  The fact that 8 buffers are available but we don’t >> use any sounds OK to me; as far as I understand, we only use those >> buffers if we have any events to push into them, so as long as we >> don’t, we won’t.  Question is, should we not have its poll handler >> return false if we don’t have any events (i.e. events_dropped is >> false)?  Would that solve it? > > Or actually, maybe we could just skip the virtio_queue_notify() call > for the event vq?  I.e. have it be `if (vq != > VIRTIO_SCSI_COMMON(s)->event_vq) { virtio_queue_notify(vdev, i); }`?  > I wouldn’t like that very much, (a) because this would make it > slightly cumbersome to put that into > virtio_queue_aio_attach_host_notifier*(), and (b) in case that does > fix it, I do kind of feel like the real problem is that we use > virtio_queue_host_notifier_aio_poll() for the event vq, which tells > the polling code to poll whenever the vq is non-empty, but we (AFAIU) > expect the event vq to be non-empty all the time. Turns out there was commit 38738f7dbbda90fbc161757b7f4be35b52205552 (“virtio-scsi: don't waste CPU polling the event virtqueue”) by Stefan, which specifically intended to not use virtio_queue_host_notifier_aio_poll() for the event vq.  I think the problem is that virtio_scsi_drained_end() should have taken care to use virtio_queue_aio_attach_host_notifier_no_poll() for the event vq. If we do that, I think running virtio_queue_notify() on the event vq too should be reasonable.  We still want to check whether there are new buffers available in case we have events_dropped.  I don’t know whether it’s really necessary, but without virtio_queue_host_notifier_aio_poll() installed, it shouldn’t hurt. Hanna