From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50916) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ewQ0h-0008IC-9k for qemu-devel@nongnu.org; Thu, 15 Mar 2018 06:21:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ewQ0d-0004HI-45 for qemu-devel@nongnu.org; Thu, 15 Mar 2018 06:21:23 -0400 Received: from mga14.intel.com ([192.55.52.115]:17302) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ewQ0c-0004CN-R6 for qemu-devel@nongnu.org; Thu, 15 Mar 2018 06:21:19 -0400 Message-ID: <5AAA49D0.7010200@intel.com> Date: Thu, 15 Mar 2018 18:24:16 +0800 From: Wei Wang MIME-Version: 1.0 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> In-Reply-To: <20180315044543-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit 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: "Michael S. Tsirkin" 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 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