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Subject: Re: [virtio-dev] Re: [PATCH v5 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
On Tue, Mar 20, 2018 at 11:18:23AM +0800, Wei Wang wrote: > 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? When threads are coming and going, they are hard to control. Consider the rich API libvirt exposes for controlling the io threads: https://libvirt.org/formatdomain.html#elementsIOThreadsAllocation > and how does BH solve it? Thanks. > Best, > Wei It's late at night so I don't remember whether it's the emulator or the io thread that runs the BH, but the point is it's already controlled by libvirt. -- MST
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