From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50060) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ey7kB-0005Jn-VS for qemu-devel@nongnu.org; Mon, 19 Mar 2018 23:15:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ey7k8-0006r8-Nv for qemu-devel@nongnu.org; Mon, 19 Mar 2018 23:15:23 -0400 Received: from mga12.intel.com ([192.55.52.136]:23873) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ey7k8-0006pq-5P for qemu-devel@nongnu.org; Mon, 19 Mar 2018 23:15:20 -0400 Message-ID: <5AB07D7F.90205@intel.com> Date: Tue, 20 Mar 2018 11:18:23 +0800 From: Wei Wang MIME-Version: 1.0 References: <1521197309-13544-1-git-send-email-wei.w.wang@intel.com> <1521197309-13544-5-git-send-email-wei.w.wang@intel.com> <20180316165319-mutt-send-email-mst@kernel.org> <5AAE4124.5010602@intel.com> <20180319062023-mutt-send-email-mst@kernel.org> <5AAF7C72.5070403@intel.com> <20180320004900-mutt-send-email-mst@kernel.org> <5AB06EE9.1030306@intel.com> <20180320045134-mutt-send-email-mst@kernel.org> In-Reply-To: <20180320045134-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 v5 4/5] 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 03/20/2018 10:59 AM, Michael S. Tsirkin wrote: > On Tue, Mar 20, 2018 at 10:16:09AM +0800, Wei Wang wrote: >> On 03/20/2018 06:55 AM, Michael S. Tsirkin wrote: >>> On Mon, Mar 19, 2018 at 05:01:38PM +0800, Wei Wang wrote: >>>> On 03/19/2018 12:24 PM, Michael S. Tsirkin wrote: >>>>> On Sun, Mar 18, 2018 at 06:36:20PM +0800, Wei Wang wrote: >>>>>> On 03/16/2018 11:16 PM, Michael S. Tsirkin wrote: >>>>>>> On Fri, Mar 16, 2018 at 06:48:28PM +0800, Wei Wang wrote: >>>>> OTOH it seems that if thread stops nothing will wake it up >>>>> whem vm is restarted. Such bahaviour change across vmstop/vmstart >>>>> is unexpected. >>>>> I do not understand why we want to increment the counter >>>>> on vm stop though. It does make sense to stop the thread >>>>> but why not resume where we left off when vm is resumed? >>>>> >>>> I'm not sure which counter we incremented. But it would be clear if we have >>>> a high level view of how it works (it is symmetric actually). Basically, we >>>> start the optimization when each round starts and stop it at the end of each >>>> round (i.e. before we do the bitmap sync), as shown below: >>>> >>>> 1) 1st Round starts --> free_page_start >>>> 2) 1st Round in progress.. >>>> 3) 1st Round ends --> free_page_stop >>>> 4) 2nd Round starts --> free_page_start >>>> 5) 2nd Round in progress.. >>>> 6) 2nd Round ends --> free_page_stop >>>> ...... >>>> >>>> For example, in 2), the VM is stopped. virtio_balloon_poll_free_page_hints >>>> finds the vq is empty (i.e. elem == NULL) and the runstate is stopped, the >>>> optimization thread exits immediately. That is, this optimization thread is >>>> gone forever (the optimization we can do for this round is done). We won't >>>> know when would the VM be woken up: >>>> A) If the VM is woken up very soon when the migration thread is still in >>>> progress of 2), then in 4) a new optimization thread (not the same one for >>>> the first round) will be created and start the optimization for the 2nd >>>> round as usual (If you have questions about 3) in this case, that >>>> free_page_stop will do nothing than just return, since the optimization >>>> thread has exited) ; >>>> B) If the VM is woken up after the whole migration has ended, there is still >>>> no point in resuming the optimization. >>>> >>>> I think this would be the simple design for the first release of this >>>> optimization. There are possibilities to improve case A) above by continuing >>>> optimization for the 1st Round as it is still in progress, but I think >>>> adding that complexity for this rare case wouldn't be worthwhile (at least >>>> for now). What would you think? >>>> >>>> >>>> Best, >>>> Wei >>> In my opinion this just makes the patch very messy. >>> >>> E.g. attempts to attach a debugger to the guest will call vmstop and >>> then behaviour changes. This is a receipe for heisenbugs which are then >>> extremely painful to debug. >>> >>> It is not really hard to make things symmetrical: >>> e.g. if you stop on vmstop then you should start on vmstart, etc. >>> And stopping thread should not involve a bunch of state >>> changes, just stop it and that's it. >>> >> "stop it" - do you mean to >> 1) make the thread exit (i.e.make virtio_balloon_poll_free_page_hints exit >> the while loop and return NULL); or >> 2) keep the thread staying in the while loop but yield running (e.g. >> sleep(1) or block on a mutex)? (or please let me know if you suggested a >> different implementation about stopping the thread) > I would say it makes more sense to make it block on something. > > BTW I still think you are engaging in premature optimization here. > What you are doing here is a "data plane for balloon". > I would make the feature work first by processing this in a BH. > Creating threads immediately opens up questions of isolation, > cgroups etc. > Could you please share more about how creating threads affects isolation and cgroup? and how does BH solve it? Thanks. Best, Wei