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. -- MST
