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Tsirkin" To: Andrey Ryabinin Cc: Jason Wang , Stefan Hajnoczi , Stefano Garzarella , Alexei Starovoitov , Daniel Borkmann , "David S. Miller" , Jakub Kicinski , Jesper Dangaard Brouer , John Fastabend , kvm , virtualization , netdev , linux-kernel , bpf@vger.kernel.org Subject: Re: [PATCH 6/6] vhost_net: use RCU callbacks instead of synchronize_rcu() Message-ID: <20211122043620-mutt-send-email-mst@kernel.org> References: <20211115153003.9140-1-arbn@yandex-team.com> <20211115153003.9140-6-arbn@yandex-team.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, Nov 19, 2021 at 02:32:05PM +0300, Andrey Ryabinin wrote: > > > On 11/16/21 8:00 AM, Jason Wang wrote: > > On Mon, Nov 15, 2021 at 11:32 PM Andrey Ryabinin wrote: > >> > >> Currently vhost_net_release() uses synchronize_rcu() to synchronize > >> freeing with vhost_zerocopy_callback(). However synchronize_rcu() > >> is quite costly operation. It take more than 10 seconds > >> to shutdown qemu launched with couple net devices like this: > >> -netdev tap,id=tap0,..,vhost=on,queues=80 > >> because we end up calling synchronize_rcu() netdev_count*queues times. > >> > >> Free vhost net structures in rcu callback instead of using > >> synchronize_rcu() to fix the problem. > > > > I admit the release code is somehow hard to understand. But I wonder > > if the following case can still happen with this: > > > > CPU 0 (vhost_dev_cleanup) CPU1 > > (vhost_net_zerocopy_callback()->vhost_work_queue()) > > if (!dev->worker) > > dev->worker = NULL > > > > wake_up_process(dev->worker) > > > > If this is true. It seems the fix is to move RCU synchronization stuff > > in vhost_net_ubuf_put_and_wait()? > > > > It all depends whether vhost_zerocopy_callback() can be called outside of vhost > thread context or not. If it can run after vhost thread stopped, than the race you > describe seems possible and the fix in commit b0c057ca7e83 ("vhost: fix a theoretical race in device cleanup") > wasn't complete. I would fix it by calling synchronize_rcu() after vhost_net_flush() > and before vhost_dev_cleanup(). > > As for the performance problem, it can be solved by replacing synchronize_rcu() with synchronize_rcu_expedited(). expedited causes a stop of IPIs though, so it's problematic to do it upon a userspace syscall. > But now I'm not sure that this race is actually exists and that synchronize_rcu() needed at all. > I did a bit of testing and I only see callback being called from vhost thread: > > vhost-3724 3733 [002] 2701.768731: probe:vhost_zerocopy_callback: (ffffffff81af8c10) > ffffffff81af8c11 vhost_zerocopy_callback+0x1 ([kernel.kallsyms]) > ffffffff81bb34f6 skb_copy_ubufs+0x256 ([kernel.kallsyms]) > ffffffff81bce621 __netif_receive_skb_core.constprop.0+0xac1 ([kernel.kallsyms]) > ffffffff81bd062d __netif_receive_skb_one_core+0x3d ([kernel.kallsyms]) > ffffffff81bd0748 netif_receive_skb+0x38 ([kernel.kallsyms]) > ffffffff819a2a1e tun_get_user+0xdce ([kernel.kallsyms]) > ffffffff819a2cf4 tun_sendmsg+0xa4 ([kernel.kallsyms]) > ffffffff81af9229 handle_tx_zerocopy+0x149 ([kernel.kallsyms]) > ffffffff81afaf05 handle_tx+0xc5 ([kernel.kallsyms]) > ffffffff81afce86 vhost_worker+0x76 ([kernel.kallsyms]) > ffffffff811581e9 kthread+0x169 ([kernel.kallsyms]) > ffffffff810018cf ret_from_fork+0x1f ([kernel.kallsyms]) > 0 [unknown] ([unknown]) > > This means that the callback can't run after kthread_stop() in vhost_dev_cleanup() and no synchronize_rcu() needed. > > I'm not confident that my quite limited testing cover all possible vhost_zerocopy_callback() callstacks.