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[79.178.18.124]) by smtp.gmail.com with ESMTPSA id n204sm6055762wma.5.2020.06.16.15.08.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Jun 2020 15:08:07 -0700 (PDT) Date: Tue, 16 Jun 2020 18:08:04 -0400 From: "Michael S. Tsirkin" To: Eugenio Perez Martin Cc: linux-kernel@vger.kernel.org, kvm list , virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, Jason Wang Subject: Re: [PATCH RFC v7 03/14] vhost: use batched get_vq_desc version Message-ID: <20200616180136-mutt-send-email-mst@kernel.org> References: <20200610113515.1497099-1-mst@redhat.com> <20200610113515.1497099-4-mst@redhat.com> <20200610111147-mutt-send-email-mst@kernel.org> <20200611072702-mutt-send-email-mst@kernel.org> <26bef3f07277b028034c019e456b4f236078c5fb.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 16, 2020 at 05:23:43PM +0200, Eugenio Perez Martin wrote: > On Mon, Jun 15, 2020 at 6:05 PM Eugenio Pérez wrote: > > > > On Thu, 2020-06-11 at 07:30 -0400, Michael S. Tsirkin wrote: > > > On Wed, Jun 10, 2020 at 06:18:32PM +0200, Eugenio Perez Martin wrote: > > > > On Wed, Jun 10, 2020 at 5:13 PM Michael S. Tsirkin wrote: > > > > > On Wed, Jun 10, 2020 at 02:37:50PM +0200, Eugenio Perez Martin wrote: > > > > > > > +/* This function returns a value > 0 if a descriptor was found, or 0 if none were found. > > > > > > > + * A negative code is returned on error. */ > > > > > > > +static int fetch_descs(struct vhost_virtqueue *vq) > > > > > > > +{ > > > > > > > + int ret; > > > > > > > + > > > > > > > + if (unlikely(vq->first_desc >= vq->ndescs)) { > > > > > > > + vq->first_desc = 0; > > > > > > > + vq->ndescs = 0; > > > > > > > + } > > > > > > > + > > > > > > > + if (vq->ndescs) > > > > > > > + return 1; > > > > > > > + > > > > > > > + for (ret = 1; > > > > > > > + ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq); > > > > > > > + ret = fetch_buf(vq)) > > > > > > > + ; > > > > > > > > > > > > (Expanding comment in V6): > > > > > > > > > > > > We get an infinite loop this way: > > > > > > * vq->ndescs == 0, so we call fetch_buf() here > > > > > > * fetch_buf gets less than vhost_vq_num_batch_descs(vq); descriptors. ret = 1 > > > > > > * This loop calls again fetch_buf, but vq->ndescs > 0 (and avail_vq == > > > > > > last_avail_vq), so it just return 1 > > > > > > > > > > That's what > > > > > [PATCH RFC v7 08/14] fixup! vhost: use batched get_vq_desc version > > > > > is supposed to fix. > > > > > > > > > > > > > Sorry, I forgot to include that fixup. > > > > > > > > With it I don't see CPU stalls, but with that version latency has > > > > increased a lot and I see packet lost: > > > > + ping -c 5 10.200.0.1 > > > > PING 10.200.0.1 (10.200.0.1) 56(84) bytes of data. > > > > > From 10.200.0.2 icmp_seq=1 Destination Host Unreachable > > > > > From 10.200.0.2 icmp_seq=2 Destination Host Unreachable > > > > > From 10.200.0.2 icmp_seq=3 Destination Host Unreachable > > > > 64 bytes from 10.200.0.1: icmp_seq=5 ttl=64 time=6848 ms > > > > > > > > --- 10.200.0.1 ping statistics --- > > > > 5 packets transmitted, 1 received, +3 errors, 80% packet loss, time 76ms > > > > rtt min/avg/max/mdev = 6848.316/6848.316/6848.316/0.000 ms, pipe 4 > > > > -- > > > > > > > > I cannot even use netperf. > > > > > > OK so that's the bug to try to find and fix I think. > > > > > > > > > > If I modify with my proposed version: > > > > + ping -c 5 10.200.0.1 > > > > PING 10.200.0.1 (10.200.0.1) 56(84) bytes of data. > > > > 64 bytes from 10.200.0.1: icmp_seq=1 ttl=64 time=7.07 ms > > > > 64 bytes from 10.200.0.1: icmp_seq=2 ttl=64 time=0.358 ms > > > > 64 bytes from 10.200.0.1: icmp_seq=3 ttl=64 time=5.35 ms > > > > 64 bytes from 10.200.0.1: icmp_seq=4 ttl=64 time=2.27 ms > > > > 64 bytes from 10.200.0.1: icmp_seq=5 ttl=64 time=0.426 ms > > > > > > Not sure which version this is. > > > > > > > [root@localhost ~]# netperf -H 10.200.0.1 -p 12865 -l 10 -t TCP_STREAM > > > > MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to > > > > 10.200.0.1 () port 0 AF_INET > > > > Recv Send Send > > > > Socket Socket Message Elapsed > > > > Size Size Size Time Throughput > > > > bytes bytes bytes secs. 10^6bits/sec > > > > > > > > 131072 16384 16384 10.01 4742.36 > > > > [root@localhost ~]# netperf -H 10.200.0.1 -p 12865 -l 10 -t UDP_STREAM > > > > MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to > > > > 10.200.0.1 () port 0 AF_INET > > > > Socket Message Elapsed Messages > > > > Size Size Time Okay Errors Throughput > > > > bytes bytes secs # # 10^6bits/sec > > > > > > > > 212992 65507 10.00 9214 0 482.83 > > > > 212992 10.00 9214 482.83 > > > > > > > > I will compare with the non-batch version for reference, but the > > > > difference between the two is noticeable. Maybe it's worth finding a > > > > good value for the if() inside fetch_buf? > > > > > > > > Thanks! > > > > > > > > > > I don't think it's performance, I think it's a bug somewhere, > > > e.g. maybe we corrupt a packet, or stall the queue, or > > > something like this. > > > > > > Let's do this, I will squash the fixups and post v8 so you can bisect > > > and then debug cleanly. > > > > Ok, so if we apply the patch proposed in v7 08/14 (Or the version 8 of the patchset sent), this is what happens: > > > > 1. Userland (virtio_test in my case) introduces just one buffer in vq, and it kicks > > 2. vhost module reaches fetch_descs, called from vhost_get_vq_desc. From there we call fetch_buf in a for loop. > > 3. The first time we call fetch_buf, it returns properly one buffer. However, the second time we call it, it returns 0 > > because vq->avail_idx == vq->last_avail_idx and vq->avail_idx == last_avail_idx code path. > > 4. fetch_descs assign ret = 0, so it returns 0. vhost_get_vq_desc will goto err, and it will signal no new buffer > > (returning vq->num). > > > > So to fix it and maintain the batching maybe we could return vq->ndescs in case ret == 0: > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index c0dfb5e3d2af..5993d4f34ca9 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -2315,7 +2327,8 @@ static int fetch_descs(struct vhost_virtqueue *vq) > > > > /* On success we expect some descs */ > > BUG_ON(ret > 0 && !vq->ndescs); > > - return ret; > > + return ret ?: vq->ndescs; I'd rather we used standard C. Also ret < 0 needs to be handled. Also - what if fetch of some descs fails but some succeeds? What do we want to do? Maybe: return vq->ndescs ? vq->ndescs : ret; > > } > > > > /* Reverse the effects of fetch_descs */ > > -- > > > > Another possibility could be to return different codes from fetch_buf, but I find the suggested modification easier. > > > > What do you think? > > > > Thanks! > > > > Hi! > > I can send a proposed RFC v9 in case it is more convenient for you. > > Thanks! Excellent, pls go ahead! And can you include the performance numbers? It's enough to test the final version. -- MST