On Mon, Apr 01, 2024 at 02:17:28PM -0300, Fabiano Rosas wrote:
> Peter Xu <pet...@redhat.com> writes:
>
> > On Fri, Mar 29, 2024 at 08:54:07AM +0000, Wang, Wei W wrote:
> >> On Friday, March 29, 2024 11:32 AM, Wang, Lei4 wrote:
> >> > When using the post-copy preemption feature to perform post-copy live
> >> > migration, the below scenario could lead to a deadlock and the migration
> >> > will
> >> > never finish:
> >> >
> >> > - Source connect() the preemption channel in postcopy_start().
> >> > - Source and the destination side TCP stack finished the 3-way handshake
> >> > thus the connection is successful.
> >> > - The destination side main thread is handling the loading of the bulk
> >> > RAM
> >> > pages thus it doesn't start to handle the pending connection event in
> >> > the
> >> > event loop. and doesn't post the semaphore postcopy_qemufile_dst_done
> >> > for
> >> > the preemption thread.
> >> > - The source side sends non-iterative device states, such as the virtio
> >> > states.
> >> > - The destination main thread starts to receive the virtio states, this
> >> > process may lead to a page fault (e.g.,
> >> > virtio_load()->vring_avail_idx()
> >> > may trigger a page fault since the avail ring page may not be received
> >> > yet).
> >
> > Ouch. Yeah I think this part got overlooked when working on the preempt
> > channel.
> >
> >> > - The page request is sent back to the source side. Source sends the
> >> > page
> >> > content to the destination side preemption thread.
> >> > - Since the event is not arrived and the semaphore
> >> > postcopy_qemufile_dst_done is not posted, the preemption thread in
> >> > destination side is blocked, and cannot handle receiving the page.
> >> > - The QEMU main load thread on the destination side is stuck at the page
> >> > fault, and cannot yield and handle the connect() event for the
> >> > preemption channel to unblock the preemption thread.
> >> > - The postcopy will stuck there forever since this is a deadlock.
> >> >
> >> > The key point to reproduce this bug is that the source side is sending
> >> > pages at a
> >> > rate faster than the destination handling, otherwise, the
> >> > qemu_get_be64() in
> >> > ram_load_precopy() will have a chance to yield since at that time there
> >> > are no
> >> > pending data in the buffer to get. This will make this bug harder to be
> >> > reproduced.
> >
> > How hard would this reproduce?
> >
> > I'm thinking whether this should be 9.0 material or 9.1. It's pretty late
> > for 9.0 though, but we can still discuss.
> >
> >> >
> >> > Fix this by yielding the load coroutine when receiving
> >> > MIG_CMD_POSTCOPY_LISTEN so the main event loop can handle the
> >> > connection event before loading the non-iterative devices state to avoid
> >> > the
> >> > deadlock condition.
> >> >
> >> > Signed-off-by: Lei Wang <lei4.w...@intel.com>
> >>
> >> This seems to be a regression issue caused by this commit:
> >> 737840e2c6ea (migration: Use the number of transferred bytes directly)
> >>
> >> Adding qemu_fflush back to migration_rate_exceeded() or ram_save_iterate
> >> seems to work (might not be a good fix though).
> >>
> >> > ---
> >> > migration/savevm.c | 5 +++++
> >> > 1 file changed, 5 insertions(+)
> >> >
> >> > diff --git a/migration/savevm.c b/migration/savevm.c index
> >> > e386c5267f..8fd4dc92f2 100644
> >> > --- a/migration/savevm.c
> >> > +++ b/migration/savevm.c
> >> > @@ -2445,6 +2445,11 @@ static int loadvm_process_command(QEMUFile *f)
> >> > return loadvm_postcopy_handle_advise(mis, len);
> >> >
> >> > case MIG_CMD_POSTCOPY_LISTEN:
> >> > + if (migrate_postcopy_preempt() && qemu_in_coroutine()) {
> >> > + aio_co_schedule(qemu_get_current_aio_context(),
> >> > + qemu_coroutine_self());
> >> > + qemu_coroutine_yield();
> >> > + }
> >>
> >> The above could be moved to loadvm_postcopy_handle_listen().
> >
> > I'm not 100% sure such thing (no matter here or moved into it, which does
> > look cleaner) would work for us.
> >
> > The problem is I still don't yet see an ordering restricted on top of (1)
> > accept() happens, and (2) receive LISTEN cmd here. What happens if the
> > accept() request is not yet received when reaching LISTEN? Or is it always
> > guaranteed the accept(fd) will always be polled here?
> >
> > For example, the source QEMU (no matter pre-7.2 or later) will always setup
> > the preempt channel asynchrounously, then IIUC it can connect() after
> > sending the whole chunk of packed data which should include this LISTEN. I
> > think it means it's not guaranteed this will 100% work, but maybe further
> > reduce the possibility of the race.
> >
> > One right fix that I can think of is moving the sem_wait(&done) into the
> > main thread too, so we wait for the sem _before_ reading the packed data,
> > so there's no chance of fault. However I don't think sem_wait() will be
> > smart enough to yield when in a coroutine.. In the long term run I think
> > we should really make migration loadvm to do work in the thread rather than
> > the main thread. I think it means we have one more example to be listed in
> > this todo so that's preferred..
> >
> > https://wiki.qemu.org/ToDo/LiveMigration#Create_a_thread_for_migration_destination
> >
> > I attached such draft patch below, but I'm not sure it'll work. Let me
> > know how both of you think about it.
> >
> >>
> >> Another option is to follow the old way (i.e. pre_7_2) to do
> >> postcopy_preempt_setup
> >> in migrate_fd_connect. This can save the above overhead of switching to the
> >> main thread during the downtime. Seems Peter's previous patch already
> >> solved the
> >> channel disordering issue. Let's see Peter and others' opinions.
> >
> > IIUC we still need that pre_7_2 stuff and keep the postponed connect() to
> > make sure the ordering is done properly. Wei, could you elaborate the
> > patch you mentioned? Maybe I missed some spots.
> >
> > You raised a good point that this may introduce higher downtime. Did you
> > or Lei tried to measure how large it is? If that is too high, we may need
> > to think another solution, e.g., wait the channel connection before vm stop
> > happens.
> >
> > Thanks,
> >
> >>
> >> > return loadvm_postcopy_handle_listen(mis);
> >> >
> >>
> >> > case MIG_CMD_POSTCOPY_RUN:
> >> > --
> >> > 2.39.3
> >>
> >
> > ===8<===
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 696762bc64..bacd1328cf 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -2593,6 +2593,12 @@ static int postcopy_start(MigrationState *ms, Error
> > **errp)
> > /*
> > * Make sure the receiver can get incoming pages before we send the
> > rest
> > * of the state
> > + *
> > + * When preempt mode enabled, this must be done after we initiate the
> > + * preempt channel, as destination QEMU will wait for the channel when
> > + * processing the LISTEN request. Currently it may not matter a huge
> > + * deal if we always create the channel asynchrously with a qio task,
> > + * but we need to keep this in mind.
> > */
> > qemu_savevm_send_postcopy_listen(fb);
> >
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index eccff499cb..4f26a89ac9 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -1254,6 +1254,26 @@ int
> > postcopy_ram_incoming_setup(MigrationIncomingState *mis)
> > }
> >
> > if (migrate_postcopy_preempt()) {
> > + /*
> > + * The preempt channel is established in asynchronous way. Wait
> > + * for its completion.
> > + */
> > + while (!qemu_sem_timedwait(&mis->postcopy_qemufile_dst_done, 100))
> > {
> > + /*
> > + * Note that to make sure the main thread can still schedule an
> > + * accept() request we need to proactively yield for the main
> > + * loop to run for some duration (100ms in this case), which is
> > + * pretty ugly.
> > + *
> > + * TODO: we should do this in a separate thread to load the VM
> > + * rather than in the main thread, just like the source side.
> > + */
> > + if (qemu_in_coroutine()) {
> > + aio_co_schedule(qemu_get_current_aio_context(),
> > + qemu_coroutine_self());
> > + qemu_coroutine_yield();
>
> I think the correct way to do this these days is
> aio_co_reschedule_self().
The helper checks old v.s. new contexts, where here we want to pass in the
current context. Would that be a no-op then?
>
> Anyway, what we are yielding to here? I see qemu_loadvm_state_main()
> called from a bunch of places, it's not clear to me where will the
> execution resume after yielding. Is that end up going to be
> migration_incoming_process()?
In this specific case it should try to yield to the port listener that is
waiting for the preempt channel, aka, socket_accept_incoming_migration(),
and ultimately it'll kick off this sem, by:
socket_accept_incoming_migration ->
migration_ioc_process_incoming ->
postcopy_preempt_new_channel
>
> I don't know much about the postcopy parts, excuse my ignorance.
Not a problem at all, please shoot if there's any questions either here or
elsewhere. You're going to maintain it anyway as part of the migration code
base. :-D
Thanks,
--
Peter Xu
