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().
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()?
I don't know much about the postcopy parts, excuse my ignorance.
> + }
> + }
> /*
> * This thread needs to be created after the temp pages because
> * it'll fetch RAM_CHANNEL_POSTCOPY PostcopyTmpPage immediately.
> @@ -1743,12 +1763,6 @@ void *postcopy_preempt_thread(void *opaque)
>
> qemu_sem_post(&mis->thread_sync_sem);
>
> - /*
> - * The preempt channel is established in asynchronous way. Wait
> - * for its completion.
> - */
> - qemu_sem_wait(&mis->postcopy_qemufile_dst_done);
> -
> /* Sending RAM_SAVE_FLAG_EOS to terminate this thread */
> qemu_mutex_lock(&mis->postcopy_prio_thread_mutex);
> while (preempt_thread_should_run(mis)) {
> --
> 2.44.0
