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Tsirkin" To: Jason Wang Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop Message-ID: <20230726073453-mutt-send-email-mst@kernel.org> References: <263a5ad7-1189-3be3-70de-c38a685bebe0@redhat.com> <20230721104445-mutt-send-email-mst@kernel.org> <6278a4aa-8901-b0e3-342f-5753a4bf32af@redhat.com> <20230721110925-mutt-send-email-mst@kernel.org> <20230724025720-mutt-send-email-mst@kernel.org> <20230725033506-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: xuanzhuo@linux.alibaba.com, edumazet@google.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Maxime Coquelin , kuba@kernel.org, pabeni@redhat.com, davem@davemloft.net 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: 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d4-20020a17090694c400b009930042510csm9538412ejy.222.2023.07.26.04.37.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 Jul 2023 04:37:49 -0700 (PDT) Date: Wed, 26 Jul 2023 07:37:24 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: Maxime Coquelin , Shannon Nelson , xuanzhuo@linux.alibaba.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, davem@davemloft.net Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop Message-ID: <20230726073453-mutt-send-email-mst@kernel.org> References: <263a5ad7-1189-3be3-70de-c38a685bebe0@redhat.com> <20230721104445-mutt-send-email-mst@kernel.org> <6278a4aa-8901-b0e3-342f-5753a4bf32af@redhat.com> <20230721110925-mutt-send-email-mst@kernel.org> <20230724025720-mutt-send-email-mst@kernel.org> <20230725033506-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 Wed, Jul 26, 2023 at 09:55:37AM +0800, Jason Wang wrote: > On Tue, Jul 25, 2023 at 3:36 PM Michael S. Tsirkin wrote: > > > > On Tue, Jul 25, 2023 at 11:07:40AM +0800, Jason Wang wrote: > > > On Mon, Jul 24, 2023 at 3:17 PM Michael S. Tsirkin wrote: > > > > > > > > On Mon, Jul 24, 2023 at 02:52:05PM +0800, Jason Wang wrote: > > > > > On Sat, Jul 22, 2023 at 4:18 AM Maxime Coquelin > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > On 7/21/23 17:10, Michael S. Tsirkin wrote: > > > > > > > On Fri, Jul 21, 2023 at 04:58:04PM +0200, Maxime Coquelin wrote: > > > > > > >> > > > > > > >> > > > > > > >> On 7/21/23 16:45, Michael S. Tsirkin wrote: > > > > > > >>> On Fri, Jul 21, 2023 at 04:37:00PM +0200, Maxime Coquelin wrote: > > > > > > >>>> > > > > > > >>>> > > > > > > >>>> On 7/20/23 23:02, Michael S. Tsirkin wrote: > > > > > > >>>>> On Thu, Jul 20, 2023 at 01:26:20PM -0700, Shannon Nelson wrote: > > > > > > >>>>>> On 7/20/23 1:38 AM, Jason Wang wrote: > > > > > > >>>>>>> > > > > > > >>>>>>> Adding cond_resched() to the command waiting loop for a better > > > > > > >>>>>>> co-operation with the scheduler. This allows to give CPU a breath to > > > > > > >>>>>>> run other task(workqueue) instead of busy looping when preemption is > > > > > > >>>>>>> not allowed on a device whose CVQ might be slow. > > > > > > >>>>>>> > > > > > > >>>>>>> Signed-off-by: Jason Wang > > > > > > >>>>>> > > > > > > >>>>>> This still leaves hung processes, but at least it doesn't pin the CPU any > > > > > > >>>>>> more. Thanks. > > > > > > >>>>>> Reviewed-by: Shannon Nelson > > > > > > >>>>>> > > > > > > >>>>> > > > > > > >>>>> I'd like to see a full solution > > > > > > >>>>> 1- block until interrupt > > > > > > > > > > I remember in previous versions, you worried about the extra MSI > > > > > vector. (Maybe I was wrong). > > > > > > > > > > > >>>> > > > > > > >>>> Would it make sense to also have a timeout? > > > > > > >>>> And when timeout expires, set FAILED bit in device status? > > > > > > >>> > > > > > > >>> virtio spec does not set any limits on the timing of vq > > > > > > >>> processing. > > > > > > >> > > > > > > >> Indeed, but I thought the driver could decide it is too long for it. > > > > > > >> > > > > > > >> The issue is we keep waiting with rtnl locked, it can quickly make the > > > > > > >> system unusable. > > > > > > > > > > > > > > if this is a problem we should find a way not to keep rtnl > > > > > > > locked indefinitely. > > > > > > > > > > Any ideas on this direction? Simply dropping rtnl during the busy loop > > > > > will result in a lot of races. This seems to require non-trivial > > > > > changes in the networking core. > > > > > > > > > > > > > > > > > From the tests I have done, I think it is. With OVS, a reconfiguration > > > > > > is performed when the VDUSE device is added, and when a MLX5 device is > > > > > > in the same bridge, it ends up doing an ioctl() that tries to take the > > > > > > rtnl lock. In this configuration, it is not possible to kill OVS because > > > > > > it is stuck trying to acquire rtnl lock for mlx5 that is held by virtio- > > > > > > net. > > > > > > > > > > Yeah, basically, any RTNL users would be blocked forever. > > > > > > > > > > And the infinite loop has other side effects like it blocks the freezer to work. > > > > > > > > > > To summarize, there are three issues > > > > > > > > > > 1) busy polling > > > > > 2) breaks freezer > > > > > 3) hold RTNL during the loop > > > > > > > > > > Solving 3 may help somehow for 2 e.g some pm routine e.g wireguard or > > > > > even virtnet_restore() itself may try to hold the lock. > > > > > > > > Yep. So my feeling currently is, the only real fix is to actually > > > > queue up the work in software. > > > > > > Do you mean something like: > > > > > > rtnl_lock(); > > > queue up the work > > > rtnl_unlock(); > > > return success; > > > > > > ? > > > > yes > > We will lose the error reporting, is it a real problem or not? Fundamental isn't it? Maybe we want a per-device flag for a asynch commands, and vduse will set it while hardware virtio won't. this way we only lose error reporting for vduse. > > > > > > > > It's mostly trivial to limit > > > > memory consumption, vid's is the > > > > only one where it would make sense to have more than > > > > 1 command of a given type outstanding. > > > > > > And rx mode so this implies we will fail any command if the previous > > > work is not finished. > > > > don't fail it, store it. > > Ok. > > Thanks > > > > > > > have a tree > > > > or a bitmap with vids to add/remove? > > > > > > Probably. > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >>>>> 2- still handle surprise removal correctly by waking in that case > > > > > > > > > > This is basically what version 1 did? > > > > > > > > > > https://lore.kernel.org/lkml/6026e801-6fda-fee9-a69b-d06a80368621@redhat.com/t/ > > > > > > > > > > Thanks > > > > > > > > Yes - except the timeout part. > > > > > > > > > > > > > > >>>>> > > > > > > >>>>> > > > > > > >>>>> > > > > > > >>>>>>> --- > > > > > > >>>>>>> drivers/net/virtio_net.c | 4 +++- > > > > > > >>>>>>> 1 file changed, 3 insertions(+), 1 deletion(-) > > > > > > >>>>>>> > > > > > > >>>>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > > > > > > >>>>>>> index 9f3b1d6ac33d..e7533f29b219 100644 > > > > > > >>>>>>> --- a/drivers/net/virtio_net.c > > > > > > >>>>>>> +++ b/drivers/net/virtio_net.c > > > > > > >>>>>>> @@ -2314,8 +2314,10 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd, > > > > > > >>>>>>> * into the hypervisor, so the request should be handled immediately. > > > > > > >>>>>>> */ > > > > > > >>>>>>> while (!virtqueue_get_buf(vi->cvq, &tmp) && > > > > > > >>>>>>> - !virtqueue_is_broken(vi->cvq)) > > > > > > >>>>>>> + !virtqueue_is_broken(vi->cvq)) { > > > > > > >>>>>>> + cond_resched(); > > > > > > >>>>>>> cpu_relax(); > > > > > > >>>>>>> + } > > > > > > >>>>>>> > > > > > > >>>>>>> return vi->ctrl->status == VIRTIO_NET_OK; > > > > > > >>>>>>> } > > > > > > >>>>>>> -- > > > > > > >>>>>>> 2.39.3 > > > > > > >>>>>>> > > > > > > >>>>>>> _______________________________________________ > > > > > > >>>>>>> Virtualization mailing list > > > > > > >>>>>>> Virtualization@lists.linux-foundation.org > > > > > > >>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization > > > > > > >>>>> > > > > > > >>> > > > > > > > > > > > > > > > > > > >