Re: [PATCH] migration: Yield coroutine when receiving MIG_CMD_POSTCOPY_LISTEN

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Mon, Apr 01, 2024 at 02:17:28PM -0300, Fabiano Rosas wrote:
>> Peter Xu <peterx@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.wang@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

Ok, I think I get it. So the issue is just a plain old "blocking the
main loop" kind of bug. We have in ram_load_precopy:

        /*
         * Yield periodically to let main loop run, but an iteration of
         * the main loop is expensive, so do it each some iterations
         */
        if ((i & 32767) == 0 && qemu_in_coroutine()) {
            aio_co_schedule(qemu_get_current_aio_context(),
                            qemu_coroutine_self());
            qemu_coroutine_yield();
        }

That's similar to why I had to move multifd_send_setup() to the
migration thread, we need to allow glib_pollfds_poll() to run so it
dispatches the listener callbacks.

>
>> 
>> 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

/me runs

But yeah, I didn't spend enough time looking at this code yet to form a
good mental picture. I only looked at the super-specific recovery cases.

>
> Thanks,