Re: [libvirt PATCH v2 81/81] RFC: qemu: Keep vCPUs paused while migration is in postcopy-paused

[copy Dave] On Mon, Jun 06, 2022 at 12:29:39PM +0100, Daniel P. Berrangé wrote: > On Wed, Jun 01, 2022 at 02:50:21PM +0200, Jiri Denemark wrote: > > QEMU keeps guest CPUs running even in postcopy-paused migration state so > > that processes that already have all memory pages they need migrated to > > the destination can keep running. However, this behavior might bring > > unexpected delays in interprocess communication as some processes will > > be stopped until migration is recover and their memory pages migrated. > > So let's make sure all guest CPUs are paused while postcopy migration is > > paused. > > --- > > > > Notes: > > Version 2: > > - new patch > > > > - this patch does not currently work as QEMU cannot handle "stop" > > QMP command while in postcopy-paused state... the monitor just > > hangs (see https://gitlab.com/qemu-project/qemu/-/issues/1052 ) > > - an ideal solution of the QEMU bug would be if QEMU itself paused > > the CPUs for us and we just got notified about it via QMP events > > - but Peter Xu thinks this behavior is actually worse than keeping > > vCPUs running > > I'd like to know what the rationale is here ? I think the wording here is definitely stronger than what I meant. :-) My understanding was stopping the VM may or may not help the guest, depending on the guest behavior at the point of migration failure. And if we're not 100% sure of that, doing nothing is the best we have, as explicitly stopping the VM is something extra we do, and it's not part of the requirements for either postcopy itself or the recovery routine. Some examples below. 1) If many of the guest threads are doing cpu intensive work, and if the needed pageset is already migrated, then stopping the vcpu threads means they could have been running during this "downtime" but we forced them not to. Actually if the postcopy didn't pause immediately right after switch, we could very possibly migrated the workload pages if the working set is not very large. 2) If we're reaching the end of the postcopy phase and it paused, most of the pages could have been migrated already. So maybe only a few or even none thread will be stopped due to remote page faults. 3) Think about kvm async page fault: that's a feature that the guest can do to yield the guest thread when there's a page fault. It means even if some of the page faulted threads got stuck for a long time due to postcopy pausing, the guest is "smart" to know it'll take a long time (userfaultfd is a major fault, and as long as KVM gup won't get the page we put the page fault into async pf queue) then the guest vcpu can explicitly schedule() the faulted context and run some other threads that may not need to be blocked. What I wanted to say is I don't know whether assuming "stopping the VM will be better than not doing so" will always be true here. If it's case by case I feel like the better way to do is to do nothing special. > > We've got a long history knowing the behaviour and impact when > pausing a VM as a whole. Of course some apps may have timeouts > that are hit if the paused time was too long, but overall this > scenario is not that different from a bare metal machine doing > suspend-to-ram. Application impact is limited & predictable and > genrally well understood. My other question is, even if we stopped the VM then right after we resume the VM won't many of those timeout()s trigger as well? I think I asked similar question to Jiri and the answer at that time was that we could have not called the timeout() function, however I think it's not persuasive enough as timeout() is the function that should take the major time so at least we're not sure whether we'll be on it already.

My understanding is that a VM can work properly after a migration because the guest timekeeping will gradually sync up with the real world time, so if there's a major donwtime triggered we can hardly make it not affecting the guest. What we can do is if we know a software is in VM context we should be robust on the timeout (and that's at least what I do on programs even on bare metal because I'd assume the program be run on an extremely busy host). But I could be all wrong on that, because I don't know enough on the whole rational of the importance of stopping the VM in the past.

> The length of outage for a CPU when post-copy transport is broken > is potentially orders of magnitude larger than the temporary > blockage while fetching a memory page asynchronously. The latter > is obviously not good for real-time sensitive apps, but most apps > and OS will cope with CPUs being stalled for 100's of milliseconds. > That isn't the case if CPUs get stalled for minutes, or even hours, > at a time due to a broken network link needing admin recovery work > in the host infra. So let me also look at the issue on having vm stop hanged, no matter whether we'd like an explicit vm_stop that hang should better be avoided from libvirt pov. Ideally it could be avoided but I need to look into it. I think it can be that the vm_stop was waiting for other vcpus to exit to userspace but those didn't really come alive after the SIG_IPI sent to them (in reality that's SIGUSR1; and I'm pretty sure all vcpu threads can handle SIGKILL.. so maybe I need to figure out where got it blocked in the kernel). I'll update either here or in the bug that Jiri opened when I got more clues out of it. Thanks, -- Peter Xu