On 4/5/2024 0:25, Wang, Wei W wrote:> 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)
When timeout, qemu_sem_timedwait() will return -1. I think the patch test passed may because you will always have at least one yield (the first yield in the do ...while ...) when loadvm_handle_cmd_packaged()? > >> >> 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); > > > > > > >