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Tsirkin" To: Jason Wang Subject: Re: [External] Re: [PATCH] virtio_ring: Suppress tx interrupt when napi_tx disable Message-ID: <20230324025937-mutt-send-email-mst@kernel.org> References: <20230321085953.24949-1-huangjie.albert@bytedance.com> <20230324013805-mutt-send-email-mst@kernel.org> <20230324024155-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: =?utf-8?B?6buE5p2w?= , open list , "open list:VIRTIO CORE AND NET DRIVERS" X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" T24gRnJpLCBNYXIgMjQsIDIwMjMgYXQgMDI6NDc6MDJQTSArMDgwMCwgSmFzb24gV2FuZyB3cm90 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cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Mar 2023 00:00:10 -0700 (PDT) Date: Fri, 24 Mar 2023 03:00:08 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: =?utf-8?B?6buE5p2w?= , "open list:VIRTIO CORE AND NET DRIVERS" , open list Subject: Re: [External] Re: [PATCH] virtio_ring: Suppress tx interrupt when napi_tx disable Message-ID: <20230324025937-mutt-send-email-mst@kernel.org> References: <20230321085953.24949-1-huangjie.albert@bytedance.com> <20230324013805-mutt-send-email-mst@kernel.org> <20230324024155-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 24, 2023 at 02:47:02PM +0800, Jason Wang wrote: > On Fri, Mar 24, 2023 at 2:42 PM Michael S. Tsirkin wrote: > > > > On Fri, Mar 24, 2023 at 02:32:40PM +0800, Jason Wang wrote: > > > On Fri, Mar 24, 2023 at 1:59 PM Michael S. Tsirkin wrote: > > > > > > > > On Fri, Mar 24, 2023 at 11:41:12AM +0800, Jason Wang wrote: > > > > > On Thu, Mar 23, 2023 at 4:01 PM 黄杰 wrote: > > > > > > > > > > > > Jason Wang 于2023年3月22日周三 10:37写道： > > > > > > > > > > > > > > On Tue, Mar 21, 2023 at 5:00 PM Albert Huang > > > > > > > wrote: > > > > > > > > > > > > > > > > From: "huangjie.albert" > > > > > > > > > > > > > > > > fix commit 8d622d21d248 ("virtio: fix up virtio_disable_cb") > > > > > > > > > > > > > > > > if we disable the napi_tx. when we triger a tx interrupt, the > > > > > > > > > > > > > > typo should be "trigger" > > > > > > > > > > > > > > > > > > > OK, thanks for this. I will correct it in the next version > > > > > > > > > > > > > > vq->event_triggered will be set to true. It will no longer be > > > > > > > > set to false. Unless we explicitly call virtqueue_enable_cb_delayed > > > > > > > > or virtqueue_enable_cb_prepare > > > > > > > > > > > > > > > > if we disable the napi_tx, It will only be called when the tx ring > > > > > > > > buffer is relatively small: > > > > > > > > virtio_net->start_xmit: > > > > > > > > if (sq->vq->num_free < 2+MAX_SKB_FRAGS) { > > > > > > > > netif_stop_subqueue(dev, qnum); > > > > > > > > if (!use_napi && > > > > > > > > unlikely(!virtqueue_enable_cb_delayed(sq->vq))) { > > > > > > > > /* More just got used, free them then recheck. */ > > > > > > > > free_old_xmit_skbs(sq, false); > > > > > > > > if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) { > > > > > > > > netif_start_subqueue(dev, qnum); > > > > > > > > virtqueue_disable_cb(sq->vq); > > > > > > > > } > > > > > > > > > > > > > > The code example here is out of date, make sure your tree has this: > > > > > > > > > > > > also, I will correct it in the next version，this is from kernel 5.15. > > > > > > > > > > > > > > > > > > > > commit d71ebe8114b4bf622804b810f5e274069060a174 > > > > > > > Author: Jason Wang > > > > > > > Date: Tue Jan 17 11:47:07 2023 +0800 > > > > > > > > > > > > > > virtio-net: correctly enable callback during start_xmit > > > > > > > > > > > > > > > } > > > > > > > > } > > > > > > > > Because event_triggered is true.Therefore, VRING_AVAIL_F_NO_INTERRUPT or > > > > > > > > VRING_PACKED_EVENT_FLAG_DISABLE will not be set.So we update > > > > > > > > vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap > > > > > > > > every time we call virtqueue_get_buf_ctx.This will bring more interruptions. > > > > > > > > > > > > > > Can you please post how to test with the performance numbers? > > > > > > > > > > > > > > > > > > > iperf3 tcp stream: > > > > > > vm1 -----------------> vm2 > > > > > > vm2 just receive tcp data stream from vm1, and send the ack to vm1, > > > > > > there are so > > > > > > many tx interruptions in vm2. > > > > > > > > > > > > but without event_triggered there are just a few tx interruptions. > > > > > > > > > > > > > > > > > > > > > > if event_triggered is set to true, do not update vring_used_event(&vq->split.vring) > > > > > > > > or vq->packed.vring.driver->off_wrap > > > > > > > > > > > > > > > > Signed-off-by: huangjie.albert > > > > > > > > --- > > > > > > > > drivers/virtio/virtio_ring.c | 6 ++++-- > > > > > > > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > > > > > index 307e139cb11d..f486cccadbeb 100644 > > > > > > > > --- a/drivers/virtio/virtio_ring.c > > > > > > > > +++ b/drivers/virtio/virtio_ring.c > > > > > > > > @@ -795,7 +795,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, > > > > > > > > /* If we expect an interrupt for the next entry, tell host > > > > > > > > * by writing event index and flush out the write before > > > > > > > > * the read in the next get_buf call. */ > > > > > > > > - if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) > > > > > > > > + if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) > > > > > > > > + && (vq->event_triggered == false)) > > > > > > > > > > > > > > I'm not sure this can work, when event_triggered is true it means > > > > > > > we've got an interrupt, in this case if we want another interrupt for > > > > > > > the next entry, we should update used_event otherwise we will lose > > > > > > > that interrupt? > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > Normally, if we receive an interrupt, we should disable the interrupt > > > > > > in the interrupt callback handler. > > > > > > > > > > So the problem is: > > > > > > > > > > 1) event_triggered was set to true in vring_interrupt() > > > > > > > > > > 2) after this nothing will happen for virtqueue_disable_cb() so > > > > > VRING_AVAIL_F_NO_INTERRUPT is not set in avail_flags_shadow > > > > > 3) virtqueue_get_buf_ctx_split() will still think the cb is enabled > > > > > then it tries to publish new event > > > > > > > > Oh. Good point! I think when I wrote up > > > > 8d622d21d248 ("virtio: fix up virtio_disable_cb") > > > > I missed this corner case. > > > > > > > > > > > > > > > > > This makes me think about whether or not we really need > > > > > event_triggered. The assumption in the virtqueue_disable_cb() seems > > > > > wrong: > > > > > > > > > > /* If device triggered an event already it won't trigger one again: > > > > > * no need to disable. > > > > > */ > > > > > if (vq->event_triggered) > > > > > return; > > > > > > > > > > This is wrong if there's no event index support. > > > > > > > > > > > > I don't get it. how does this get triggered? > > > > > > > > You are talking about device without event index? > > > > Here's code from vring_interrupt(): > > > > > > > > /* Just a hint for performance: so it's ok that this can be racy! */ > > > > if (vq->event) > > > > vq->event_triggered = true; > > > > > > But we have the following in virtqueue_disable_cb(): > > > > > > /* If device triggered an event already it won't trigger one again: > > > * no need to disable. > > > */ > > > if (vq->event_triggered) > > > return; > > > > > > if (vq->packed_ring) > > > virtqueue_disable_cb_packed(_vq); > > > else > > > virtqueue_disable_cb_split(_vq); > > > > > > This means, without an event index, we don't set avail flags. So the > > > interrupt is not disabled actually in this case. > > > > > > Thanks > > > > Only if event_triggered is true, which without event index it never is. > > I'm not sure I will get here. I meant for example the commit > suppresses the effort of skb_xmit_done(): > > static void skb_xmit_done(struct virtqueue *vq) > { > struct virtnet_info *vi = vq->vdev->priv; > struct napi_struct *napi = &vi->sq[vq2txq(vq)].napi; > > /* Suppress further interrupts. */ > virtqueue_disable_cb(vq); > > The virtqueue_disable_cb() doesn't disable further interrupts when the > event index is not there. > > Thanks Check what can set event_triggered, you will see. > > > > > > > > > > > > > > > > > > > > > > > And the > > > > > event_triggered is somehow duplicated with the > > > > > VRING_AVAIL_F_NO_INTERRUPT in the case of event index. The correct fix > > > > > might be: > > > > > > > > > > 1) remove event_triggered > > > > > 2) set VRING_AVAIL_F_NO_INTERRUPT in avail_flags_shadow in > > > > > vring_interrrupt if event index is supported > > > > > > > > > > ? > > > > > > > > > > Thanks > > > > > > > > I am not sure all this is right and I'd rather we focused > > > > performance/correctness and cleanups separately. > > > > > > > > > > > > > > > > > > > > > > > > > > > But because of the introduction of event_triggered, here, > > > > > > virtqueue_get_buf_ctx_split cannot be recognized > > > > > > that the interrupt has been turned off. > > > > > > > > > > > > if we want another interrupt for the next entry, We should probably > > > > > > call virtqueue_enable_cb？ > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > virtio_store_mb(vq->weak_barriers, > > > > > > > > &vring_used_event(&vq->split.vring), > > > > > > > > cpu_to_virtio16(_vq->vdev, vq->last_used_idx)); > > > > > > > > @@ -1529,7 +1530,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, > > > > > > > > * by writing event index and flush out the write before > > > > > > > > * the read in the next get_buf call. > > > > > > > > */ > > > > > > > > - if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC) > > > > > > > > + if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC > > > > > > > > + && (vq->event_triggered == false)) > > > > > > > > virtio_store_mb(vq->weak_barriers, > > > > > > > > &vq->packed.vring.driver->off_wrap, > > > > > > > > cpu_to_le16(vq->last_used_idx)); > > > > > > > > -- > > > > > > > > 2.31.1 > > > > > > > > > > > > > > > > > > > > > > > > > > >