From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: [PATCH] vhost: locking/rcu cleanup Date: Fri, 30 Jul 2010 16:49:54 +0200 Message-ID: <4C52E692.3070405@kernel.org> References: <20100729122325.GA24337@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , Sridhar Samudrala , Jeff Dike , Juan Quintela , Rusty Russell , Takuya Yoshikawa , David Stevens , "Paul E. McKenney" , kvm@vger.kernel.org, virtualization@lists.osdl.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20100729122325.GA24337@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Hello, On 07/29/2010 02:23 PM, Michael S. Tsirkin wrote: > I saw WARN_ON(!list_empty(&dev->work_list)) trigger > so our custom flush is not as airtight as need be. Could be but it's also possible that something has queued something after the last flush? Is the problem reproducible? > This patch switches to a simple atomic counter + srcu instead of > the custom locked queue + flush implementation. > > This will slow down the setup ioctls, which should not matter - > it's slow path anyway. We use the expedited flush to at least > make sure it has a sane time bound. > > Works fine for me. I got reports that with many guests, > work lock is highly contended, and this patch should in theory > fix this as well - but I haven't tested this yet. Hmmm... vhost_poll_flush() becomes synchronize_srcu_expedited(). Can you please explain how it works? synchronize_srcu_expedited() is an extremely heavy operation involving scheduling the cpu_stop task on all cpus. I'm not quite sure whether doing it from every flush is a good idea. Is flush supposed to be a very rare operation? Having custom implementation is fine too but let's try to implement something generic if at all possible. Thanks. -- tejun