On Thursday, April 4, 2024 10:12 PM, Peter Xu wrote:
> On Thu, Apr 04, 2024 at 06:05:50PM +0800, Wei Wang wrote:
> > Before loading the guest states, ensure that the preempt channel has
> > been ready to use, as some of the states (e.g. via virtio_load) might
> > trigger page faults that will be handled through the preempt channel.
> > So yield to the main thread in the case that the channel create event
> > has been dispatched.
> >
> > Originally-by: Lei Wang <lei4.w...@intel.com>
> > Link:
> > https://lore.kernel.org/all/9aa5d1be-7801-40dd-83fd-f7e041ced249@intel
> > .com/T/
> > Suggested-by: Peter Xu <pet...@redhat.com>
> > Signed-off-by: Lei Wang <lei4.w...@intel.com>
> > Signed-off-by: Wei Wang <wei.w.w...@intel.com>
> > ---
> > migration/savevm.c | 17 +++++++++++++++++
> > 1 file changed, 17 insertions(+)
> >
> > diff --git a/migration/savevm.c b/migration/savevm.c index
> > 388d7af7cd..fbc9f2bdd4 100644
> > --- a/migration/savevm.c
> > +++ b/migration/savevm.c
> > @@ -2342,6 +2342,23 @@ static int
> > loadvm_handle_cmd_packaged(MigrationIncomingState *mis)
> >
> > QEMUFile *packf = qemu_file_new_input(QIO_CHANNEL(bioc));
> >
> > + /*
> > + * Before loading the guest states, ensure that the preempt channel has
> > + * been ready to use, as some of the states (e.g. via virtio_load)
> > might
> > + * trigger page faults that will be handled through the preempt
> > channel.
> > + * So yield to the main thread in the case that the channel create
> > event
> > + * has been dispatched.
> > + */
> > + do {
> > + if (!migrate_postcopy_preempt() || !qemu_in_coroutine() ||
> > + mis->postcopy_qemufile_dst) {
> > + break;
> > + }
> > +
> > + aio_co_schedule(qemu_get_current_aio_context(),
> qemu_coroutine_self());
> > + qemu_coroutine_yield();
> > + } while (!qemu_sem_timedwait(&mis->postcopy_qemufile_dst_done,
> > + 1));
>
> I think we need s/!// here, so the same mistake I made? I think we need to
> rework the retval of qemu_sem_timedwait() at some point later..
No. qemu_sem_timedwait returns false when timeout, which means sem isn’t posted
yet.
So it needs to go back to the loop. (the patch was tested)
>
> Besides, this patch kept the sem_wait() in postcopy_preempt_thread() so it
> will wait() on this sem again. If this qemu_sem_timedwait() accidentally
> consumed the sem count then I think the other thread can hang forever?
I can get the issue you mentioned, and seems better to be placed before the
creation of
the preempt thread. Then we probably don’t need to wait_sem in the preempt
thread, as the
channel is guaranteed to be ready when it runs?
Update will be:
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index eccff499cb..5a70ce4f23 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -1254,6 +1254,15 @@ int postcopy_ram_incoming_setup(MigrationIncomingState
*mis)
}
if (migrate_postcopy_preempt()) {
+ do {
+ if (!migrate_postcopy_preempt() || !qemu_in_coroutine() ||
+ mis->postcopy_qemufile_dst) {
+ break;
+ }
+ aio_co_schedule(qemu_get_current_aio_context(),
qemu_coroutine_self());
+ qemu_coroutine_yield();
+ } while (!qemu_sem_timedwait(&mis->postcopy_qemufile_dst_done, 1));
+
/*
* This thread needs to be created after the temp pages because
* it'll fetch RAM_CHANNEL_POSTCOPY PostcopyTmpPage immediately.
@@ -1743,12 +1752,6 @@ 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);
