On Thu, May 30, 2019 at 11:13:41AM +1000, David Gibson wrote: > On Thu, May 23, 2019 at 05:18:51PM -0300, Maxiwell S. Garcia wrote: > > On Thu, May 23, 2019 at 09:29:52AM +1000, David Gibson wrote: > > > On Mon, May 20, 2019 at 05:43:40PM -0300, Maxiwell S. Garcia wrote: > > > > This handler was added in the commit: > > > > 42043e4f1241: spapr: clock should count only if vm is running > > > > > > > > In a scenario without migration, this pre_save handler is not > > > > triggered, so the 'stop/cont' commands save and restore the clock > > > > in the function 'cpu_ppc_clock_vm_state_change.' The SW clock > > > > in the guest doesn't know about this pause. > > > > > > > > If the command 'migrate' is called between 'stop' and 'cont', > > > > the pre_save handler re-read the clock, and the SW clock in the > > > > guest will know about the pause between 'stop' and 'migrate.' > > > > If the guest is running a workload like HTC, a side-effect of > > > > this is a lot of process stall messages (with call traces) in > > > > the kernel guest. > > > > > > > > Signed-off-by: Maxiwell S. Garcia <maxiw...@linux.ibm.com> > > > > > > What affect will this have on the clock for the case of migrations > > > without a stop/cont around? > > > > The guest timebase is saved when the VM stop running and restored when > > the VM starts running again (cpu_ppc_clock_vm_state_change handler). > > Migrations without stop/cont save the clock when the VM go to the > > FINISH_MIGRATE state. > > Right... which means the clock is effectively stopped for the > migration downtime window while we transfer the final state. That > means the guest clock will drift from wall clock by a couple of > hundred ms across the migration which is not correct. > > > > The complicated thing here is that for > > > *explicit* stops/continues we want to freeze the clock, however for > > > the implicit stop/continue during migration downtime, we want to keep > > > the clock running (logically), so that the guest time of day doesn't > > > get out of sync on migration. > > > > > > > Not sure if the *implicit* word here is about commands from the libvirt > > or any other orchestrator. > > By implicit I mean the stopping of the VM which qemu does to transfer > the final part of the state, rather than because of an explicit > stop/cont command. > > > QEMU itself doesn't know the intent behind the > > command stop/cont. So, If we are using a guest to process a workload and > > the manager tool decide to migrate our VM transparently, it's unpleasant > > to see a lot of process stalls with call traces in the kernel log. > > If you have a lot of process stalls across a migration, that suggests > your permitted downtime window is *way* too long. >
I see a difference between live migration and 'cold' migration. In a cold migration scenario (where a user 'stop' the machine, wait an arbitrary time, move it to another server, and run it), the behavior should be the same as executing 'stop/cont' in a guest without migration. This problem also emerges when the 'timeout' flag is used with 'virsh' tool to live-migrate a guest. After the 'timeout', the libvirt sends a 'stop' command to QEMU to suspend the guest before migrate. If the NFS is slow, for example, this guest will wait many minutes to run again. Maybe a solution is to modify the timebase_pre_save handler to know the current vm_state and only save the timebase again when vm_state is not in 'stop' state. What do you think? > > The high-level tools could sync the SW clock with the HW clock if this > > behavior is required, keeping the QEMU stop/cont and stop/migrate/cont > > consistent. > > > > > > --- > > > > hw/ppc/ppc.c | 24 ------------------------ > > > > 1 file changed, 24 deletions(-) > > > > > > > > diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c > > > > index ad20584f26..3fb50cbeee 100644 > > > > --- a/hw/ppc/ppc.c > > > > +++ b/hw/ppc/ppc.c > > > > @@ -1056,35 +1056,11 @@ void cpu_ppc_clock_vm_state_change(void > > > > *opaque, int running, > > > > } > > > > } > > > > > > > > -/* > > > > - * When migrating, read the clock just before migration, > > > > - * so that the guest clock counts during the events > > > > - * between: > > > > - * > > > > - * * vm_stop() > > > > - * * > > > > - * * pre_save() > > > > - * > > > > - * This reduces clock difference on migration from 5s > > > > - * to 0.1s (when max_downtime == 5s), because sending the > > > > - * final pages of memory (which happens between vm_stop() > > > > - * and pre_save()) takes max_downtime. > > > > > > Urgh.. this comment is confusing - 5s would be a ludicrously long > > > max_downtime by modern standards. > > > > > > > - */ > > > > -static int timebase_pre_save(void *opaque) > > > > -{ > > > > - PPCTimebase *tb = opaque; > > > > - > > > > - timebase_save(tb); > > > > - > > > > - return 0; > > > > -} > > > > - > > > > const VMStateDescription vmstate_ppc_timebase = { > > > > .name = "timebase", > > > > .version_id = 1, > > > > .minimum_version_id = 1, > > > > .minimum_version_id_old = 1, > > > > - .pre_save = timebase_pre_save, > > > > .fields = (VMStateField []) { > > > > VMSTATE_UINT64(guest_timebase, PPCTimebase), > > > > VMSTATE_INT64(time_of_the_day_ns, PPCTimebase), > > > > > > > > > -- > David Gibson | I'll have my music baroque, and my code > david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ > _other_ > | _way_ _around_! > http://www.ozlabs.org/~dgibson