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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: <20230811054859-mutt-send-email-mst@kernel.org> References: <20230727054300-mutt-send-email-mst@kernel.org> <20230810153744-mutt-send-email-mst@kernel.org> <20230811012147-mutt-send-email-mst@kernel.org> <20230811052102-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: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 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([2.55.27.97]) by smtp.gmail.com with ESMTPSA id y24-20020a05600c365800b003fe2b081661sm7520672wmq.30.2023.08.11.02.51.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Aug 2023 02:51:43 -0700 (PDT) Date: Fri, 11 Aug 2023 05:51:39 -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: <20230811054859-mutt-send-email-mst@kernel.org> References: <20230727054300-mutt-send-email-mst@kernel.org> <20230810153744-mutt-send-email-mst@kernel.org> <20230811012147-mutt-send-email-mst@kernel.org> <20230811052102-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, Aug 11, 2023 at 05:43:25PM +0800, Jason Wang wrote: > On Fri, Aug 11, 2023 at 5:21 PM Michael S. Tsirkin wrote: > > > > On Fri, Aug 11, 2023 at 05:18:51PM +0800, Jason Wang wrote: > > > On Fri, Aug 11, 2023 at 1:42 PM Michael S. Tsirkin wrote: > > > > > > > > On Fri, Aug 11, 2023 at 10:23:15AM +0800, Jason Wang wrote: > > > > > On Fri, Aug 11, 2023 at 3:41 AM Michael S. Tsirkin wrote: > > > > > > > > > > > > On Tue, Aug 08, 2023 at 10:30:56AM +0800, Jason Wang wrote: > > > > > > > On Mon, Jul 31, 2023 at 2:30 PM Jason Wang wrote: > > > > > > > > > > > > > > > > On Thu, Jul 27, 2023 at 5:46 PM Michael S. Tsirkin wrote: > > > > > > > > > > > > > > > > > > On Thu, Jul 27, 2023 at 04:59:33PM +0800, Jason Wang wrote: > > > > > > > > > > > They really shouldn't - any NIC that takes forever to > > > > > > > > > > > program will create issues in the networking stack. > > > > > > > > > > > > > > > > > > > > Unfortunately, it's not rare as the device/cvq could be implemented > > > > > > > > > > via firmware or software. > > > > > > > > > > > > > > > > > > Currently that mean one either has sane firmware with a scheduler that > > > > > > > > > can meet deadlines, or loses ability to report errors back. > > > > > > > > > > > > > > > > > > > > But if they do they can always set this flag too. > > > > > > > > > > > > > > > > > > > > This may have false negatives and may confuse the management. > > > > > > > > > > > > > > > > > > > > Maybe we can extend the networking core to allow some device specific > > > > > > > > > > configurations to be done with device specific lock without rtnl. For > > > > > > > > > > example, split the set_channels to > > > > > > > > > > > > > > > > > > > > pre_set_channels > > > > > > > > > > set_channels > > > > > > > > > > post_set_channels > > > > > > > > > > > > > > > > > > > > The device specific part could be done in pre and post without a rtnl lock? > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > Would the benefit be that errors can be reported to userspace then? > > > > > > > > > Then maybe. I think you will have to show how this works for at least > > > > > > > > > one card besides virtio. > > > > > > > > > > > > > > > > Even for virtio, this seems not easy, as e.g the > > > > > > > > virtnet_send_command() and netif_set_real_num_tx_queues() need to > > > > > > > > appear to be atomic to the networking core. > > > > > > > > > > > > > > > > I wonder if we can re-consider the way of a timeout here and choose a > > > > > > > > sane value as a start. > > > > > > > > > > > > > > Michael, any more input on this? > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > I think this is just mission creep. We are trying to fix > > > > > > vduse - let's do that for starters. > > > > > > > > > > > > Recovering from firmware timeouts is far from trivial and > > > > > > just assuming that just because it timed out it will not > > > > > > access memory is just as likely to cause memory corruption > > > > > > with worse results than an infinite spin. > > > > > > > > > > Yes, this might require support not only in the driver > > > > > > > > > > > > > > > > > I propose we fix this for vduse and assume hardware/firmware > > > > > > is well behaved. > > > > > > > > > > One major case is the re-connection, in that case it might take > > > > > whatever longer that the kernel virito-net driver expects. > > > > > So we can have a timeout in VDUSE and trap CVQ then VDUSE can return > > > > > and fail early? > > > > > > > > Ugh more mission creep. not at all my point. vduse should cache > > > > values in the driver, > > > > > > What do you mean by values here? The cvq command? > > > > > > Thanks > > > > The card status generally. > > Just to make sure I understand here. The CVQ needs to be processed by > the userspace now. How could we cache the status? > > Thanks vduse will have to process it in kernel. > > > > > > until someone manages to change > > > > net core to be more friendly to userspace devices. > > > > > > > > > > > > > > > Or maybe not well behaved firmware will > > > > > > set the flag losing error reporting ability. > > > > > > > > > > This might be hard since it means not only the set but also the get is > > > > > unreliable. > > > > > > > > > > Thanks > > > > > > > > /me shrugs > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > MST > > > > > > > > > > > > > > > > > > > > >