From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55859) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ewTKS-00077u-KF for qemu-devel@nongnu.org; Thu, 15 Mar 2018 09:54:01 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ewTKP-0000ZV-F9 for qemu-devel@nongnu.org; Thu, 15 Mar 2018 09:54:00 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:44792 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ewTKP-0000Z0-9V for qemu-devel@nongnu.org; Thu, 15 Mar 2018 09:53:57 -0400 Date: Thu, 15 Mar 2018 15:53:56 +0200 From: "Michael S. Tsirkin" Message-ID: <20180315155051-mutt-send-email-mst@kernel.org> References: <1520426065-40265-1-git-send-email-wei.w.wang@intel.com> <1520426065-40265-4-git-send-email-wei.w.wang@intel.com> <20180313183558-mutt-send-email-mst@kernel.org> <5AA88C35.90300@intel.com> <20180314045130-mutt-send-email-mst@kernel.org> <5AA8BB27.6030205@intel.com> <20180314160832-mutt-send-email-mst@kernel.org> <5AA9C944.2010403@intel.com> <20180315044543-mutt-send-email-mst@kernel.org> <5AAA49D0.7010200@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5AAA49D0.7010200@intel.com> Subject: Re: [Qemu-devel] [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wei Wang Cc: qemu-devel@nongnu.org, virtio-dev@lists.oasis-open.org, quintela@redhat.com, dgilbert@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu0@gmail.com, nilal@redhat.com, riel@redhat.com On Thu, Mar 15, 2018 at 06:24:16PM +0800, Wei Wang wrote: > On 03/15/2018 10:47 AM, Michael S. Tsirkin wrote: > > On Thu, Mar 15, 2018 at 09:15:48AM +0800, Wei Wang wrote: > > > On 03/14/2018 10:12 PM, Michael S. Tsirkin wrote: > > > > On Wed, Mar 14, 2018 at 02:03:19PM +0800, Wei Wang wrote: > > > > > On 03/14/2018 10:53 AM, Michael S. Tsirkin wrote: > > > > > > On Wed, Mar 14, 2018 at 10:43:01AM +0800, Wei Wang wrote: > > > > > > > On 03/14/2018 12:49 AM, Michael S. Tsirkin wrote: > > > > > > > > On Wed, Mar 07, 2018 at 08:34:24PM +0800, Wei Wang wrote: > > > > > > > > > > > > > > > > > Signed-off-by: Wei Wang > > > > > > > > > Signed-off-by: Liang Li > > > > > > > > > CC: Michael S. Tsirkin > > > > > > > > > CC: Dr. David Alan Gilbert > > > > > > > > > CC: Juan Quintela > > > > > > > > I find it suspicious that neither unrealize nor reset > > > > > > > > functions have been touched at all. > > > > > > > > Are you sure you have thought through scenarious like > > > > > > > > hot-unplug or disabling the device by guest? > > > > > > > OK. I think we can call balloon_free_page_stop in unrealize and reset. > > > > > > > > > > > > > > > > > > > > > > +static void *virtio_balloon_poll_free_page_hints(void *opaque) > > > > > > > > +{ > > > > > > > > + VirtQueueElement *elem; > > > > > > > > + VirtIOBalloon *dev = opaque; > > > > > > > > + VirtQueue *vq = dev->free_page_vq; > > > > > > > > + uint32_t id; > > > > > > > > + size_t size; > > > > > > > > What makes it safe to poke at this device from multiple threads? > > > > > > > > I think that it would be safer to do it from e.g. BH. > > > > > > > > > > > > > > > Actually the free_page_optimization thread is the only user of free_page_vq, > > > > > > > and there is only one optimization thread each time. Would this be safe > > > > > > > enough? > > > > > > > > > > > > > > Best, > > > > > > > Wei > > > > > > Aren't there other fields there? Also things like reset affect all VQs. > > > > > > > > > > > Yes. But I think BHs are used to avoid re-entrancy, which isn't the issue > > > > > here. > > > > Since you are adding locks to address the issue - doesn't this imply > > > > reentrancy is exactly the issue? > > > Not really. The lock isn't intended for any reentrancy issues, since there > > > will be only one run of the virtio_balloon_poll_free_page_hints function at > > > any given time. Instead, the lock is used to synchronize > > > virtio_balloon_poll_free_page_hints and virtio_balloon_free_page_stop to > > > access dev->free_page_report_status. > > I wonder whether that's enough. E.g. is there a race with guest > > trying to reset the device? That resets all VQs you know. > > I think that's OK - we will call virtio_balloon_free_page_stop in the device > reset function, and qemu_thread_join() in virtio_balloon_free_page_stop will > wait till the optimization thread exits. That is, the reset will proceed > after the optimization thread exits. > > > > > > > > > Please see the whole picture below: > > > > > > virtio_balloon_poll_free_page_hints() > > > { > > > > > > while (1) { > > > qemu_spin_lock(); > > > if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP || > > > !runstate_is_running()) { > > > qemu_spin_unlock(); > > > break; > > > } > > > ... > > > if (id == dev->free_page_report_cmd_id) { > > > ==> dev->free_page_report_status = FREE_PAGE_REPORT_S_START; > > > ... > > > qemu_spin_unlock(); > > > } > > > } > > > > > > > > > static void virtio_balloon_free_page_stop(void *opaque) > > > { > > > VirtIOBalloon *s = opaque; > > > VirtIODevice *vdev = VIRTIO_DEVICE(s); > > > > > > qemu_spin_lock(); > > > ... > > > ==> s->free_page_report_status = FREE_PAGE_REPORT_S_STOP; > > > ... > > > qemu_spin_unlock(); > > > } > > > > > > > > > Without the lock, there are theoretical possibilities that assigning STOP > > > below is overridden by START above. In that > > > case,virtio_balloon_free_page_stop does not effectively stop > > > virtio_balloon_poll_free_page_hints. > > > I think this issue couldn't be solved by BHs. > > > > > > Best, > > > Wei > > Don't all BHs run under the BQL? > > Actually the virtio_balloon_free_page_stop is called by the migration thread > (instead of a BH). Even we guarantee the migration thread calls > virtio_balloon_free_page_stop under BQL, the BQL is still too big for our > case. Imagine this case: when the migration thread calls > virtio_balloon_free_page_stop to stop the reporting, it blocks by BQL as > virtio_balloon_poll_free_page_hints is in progress with BQL held, and the > migration thread won't proceed untill virtio_balloon_poll_free_page_hints > exits (i.e. getting all the hints). I think this isn't our intention - we > basically want the migration thread to stop the guest reporting immediately. > So I think the small lock above is better (it locks for only one hint). > > Best, > Wei I am just saying that all these locking and ownership tricks need to be documented. BH would be much simpler. -- MST