* Peter Xu (pet...@redhat.com) wrote: > On Wed, Feb 23, 2022 at 09:52:08AM +0000, Dr. David Alan Gilbert wrote: > > * Peter Xu (pet...@redhat.com) wrote: > > > On Tue, Feb 22, 2022 at 11:32:10AM +0000, Dr. David Alan Gilbert wrote: > > > > * Peter Xu (pet...@redhat.com) wrote: > > > > > To allow postcopy recovery, the ram fast load (preempt-only) dest > > > > > QEMU thread > > > > > needs similar handling on fault tolerance. When ram_load_postcopy() > > > > > fails, > > > > > instead of stopping the thread it halts with a semaphore, preparing > > > > > to be > > > > > kicked again when recovery is detected. > > > > > > > > > > A mutex is introduced to make sure there's no concurrent operation > > > > > upon the > > > > > socket. To make it simple, the fast ram load thread will take the > > > > > mutex during > > > > > its whole procedure, and only release it if it's paused. The > > > > > fast-path socket > > > > > will be properly released by the main loading thread safely when > > > > > there's > > > > > network failures during postcopy with that mutex held. > > > > > > > > I *think* this is mostly OK; but I worry I don't understand all the > > > > cases; e.g. > > > > a) If the postcopy channel errors first > > > > b) If the main channel errors first > > > > > > Ah right, I don't think I handled all the cases. Sorry. > > > > > > We always check the main channel, but if the postcopy channel got faulted, > > > we may not fall into paused mode as expected. > > > > > > I'll fix that up. > > > > Thanks. > > > > > > > > > > Can you add some docs to walk through those and explain the locking ? > > > > > > Sure. > > > > > > The sem is mentioned in the last sentence of paragraph 1, where it's > > > purely > > > used for a way to yield the fast ram load thread so that when something > > > wrong happens it can sleep on that semaphore. Then when we recover we'll > > > post to the semaphore to kick it up. We used it like that in many places, > > > e.g. postcopy_pause_sem_dst to yield the main load thread. > > > > > > The 2nd paragraph above was for explaining why we need the mutex; it's > > > basically the same as rp_mutex protecting to_src_file, so that we won't > > > accidentally close() the qemufile during some other thread using it. So > > > the fast ram load thread needs to take that new mutex for mostly the whole > > > lifecycle of itself (because it's loading from that qemufile), meanwhile > > > only drop the mutex when it prepares to sleep. Then the main load thread > > > can recycle the postcopy channel using qemu_fclose() safely. > > > > Yes, that feels like it needs to go in the code somewhere. > > Sure, I'll further squash below comment update into the same patch. I > reworded some places but mostly it should be telling the same thing: > > ---8<--- > diff --git a/migration/migration.h b/migration/migration.h > index 945088064a..91f845e9e4 100644 > --- a/migration/migration.h > +++ b/migration/migration.h > @@ -118,7 +118,17 @@ struct MigrationIncomingState { > /* Postcopy priority thread is used to receive postcopy requested pages > */ > QemuThread postcopy_prio_thread; > bool postcopy_prio_thread_created; > - /* Used to sync with the prio thread */ > + /* > + * Used to sync between the ram load main thread and the fast ram load > + * thread. It protects postcopy_qemufile_dst, which is the postcopy > + * fast channel. > + * > + * The ram fast load thread will take it mostly for the whole lifecycle > + * because it needs to continuously read data from the channel, and > + * it'll only release this mutex if postcopy is interrupted, so that > + * the ram load main thread will take this mutex over and properly > + * release the broken channel. > + */ > QemuMutex postcopy_prio_thread_mutex; > /* > * An array of temp host huge pages to be used, one for each postcopy > @@ -149,6 +159,12 @@ struct MigrationIncomingState { > /* notify PAUSED postcopy incoming migrations to try to continue */ > QemuSemaphore postcopy_pause_sem_dst; > QemuSemaphore postcopy_pause_sem_fault; > + /* > + * This semaphore is used to allow the ram fast load thread (only when > + * postcopy preempt is enabled) fall into sleep when there's network > + * interruption detected. When the recovery is done, the main load > + * thread will kick the fast ram load thread using this semaphore. > + */ > QemuSemaphore postcopy_pause_sem_fast_load;
Acked-by: Dr. David Alan Gilbert <dgilb...@redhat.com> > > /* List of listening socket addresses */ > ---8<--- > > > > > > [...] > > > > > > > > @@ -3466,6 +3468,17 @@ static MigThrError > > > > > postcopy_pause(MigrationState *s) > > > > > qemu_file_shutdown(file); > > > > > qemu_fclose(file); > > > > > > > > > > + /* > > > > > + * Do the same to postcopy fast path socket too if there is. > > > > > No > > > > > + * locking needed because no racer as long as we do this > > > > > before setting > > > > > + * status to paused. > > > > > + */ > > > > > + if (s->postcopy_qemufile_src) { > > > > > + > > > > > migration_ioc_unregister_yank_from_file(s->postcopy_qemufile_src); > > > > > > > > Shouldn't this do a qemu_file_shutdown on here first? > > > > > > Yes I probably should. > > > > > > With all above, I plan to squash below changes into this patch: > > > > > > ---8<--- > > > diff --git a/migration/migration.c b/migration/migration.c > > > index c68a281406..69778cab23 100644 > > > --- a/migration/migration.c > > > +++ b/migration/migration.c > > > @@ -3475,6 +3475,7 @@ static MigThrError postcopy_pause(MigrationState *s) > > > */ > > > if (s->postcopy_qemufile_src) { > > > > > > migration_ioc_unregister_yank_from_file(s->postcopy_qemufile_src); > > > + qemu_file_shutdown(s->postcopy_qemufile_src); > > > qemu_fclose(s->postcopy_qemufile_src); > > > s->postcopy_qemufile_src = NULL; > > > } > > > @@ -3534,8 +3535,13 @@ static MigThrError > > > migration_detect_error(MigrationState *s) > > > return MIG_THR_ERR_FATAL; > > > } > > > > > > - /* Try to detect any file errors */ > > > - ret = qemu_file_get_error_obj(s->to_dst_file, &local_error); > > > + /* > > > + * Try to detect any file errors. Note that postcopy_qemufile_src > > > will > > > + * be NULL when postcopy preempt is not enabled. > > > + */ > > > + ret = qemu_file_get_error_obj_any(s->to_dst_file, > > > + s->postcopy_qemufile_src, > > > + &local_error); > > > if (!ret) { > > > /* Everything is fine */ > > > assert(!local_error); > > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c > > > index 1479cddad9..397652f0ba 100644 > > > --- a/migration/qemu-file.c > > > +++ b/migration/qemu-file.c > > > @@ -139,6 +139,33 @@ int qemu_file_get_error_obj(QEMUFile *f, Error > > > **errp) > > > return f->last_error; > > > } > > > > > > +/* > > > + * Get last error for either stream f1 or f2 with optional Error*. > > > + * The error returned (non-zero) can be either from f1 or f2. > > > + * > > > + * If any of the qemufile* is NULL, then skip the check on that file. > > > + * > > > + * When there is no error on both qemufile, zero is returned. > > > + */ > > > +int qemu_file_get_error_obj_any(QEMUFile *f1, QEMUFile *f2, Error **errp) > > > +{ > > > + int ret = 0; > > > + > > > + if (f1) { > > > + ret = qemu_file_get_error_obj(f1, errp); > > > + /* If there's already error detected, return */ > > > + if (ret) { > > > + return ret; > > > + } > > > + } > > > + > > > + if (f2) { > > > + ret = qemu_file_get_error_obj(f2, errp); > > > + } > > > + > > > + return ret; > > > +} > > > + > > > /* > > > * Set the last error for stream f with optional Error* > > > */ > > > diff --git a/migration/qemu-file.h b/migration/qemu-file.h > > > index 3f36d4dc8c..2564e5e1c7 100644 > > > --- a/migration/qemu-file.h > > > +++ b/migration/qemu-file.h > > > @@ -156,6 +156,7 @@ void qemu_file_update_transfer(QEMUFile *f, int64_t > > > len); > > > void qemu_file_set_rate_limit(QEMUFile *f, int64_t new_rate); > > > int64_t qemu_file_get_rate_limit(QEMUFile *f); > > > int qemu_file_get_error_obj(QEMUFile *f, Error **errp); > > > +int qemu_file_get_error_obj_any(QEMUFile *f1, QEMUFile *f2, Error > > > **errp); > > > void qemu_file_set_error_obj(QEMUFile *f, int ret, Error *err); > > > void qemu_file_set_error(QEMUFile *f, int ret); > > > int qemu_file_shutdown(QEMUFile *f); > > > diff --git a/migration/savevm.c b/migration/savevm.c > > > index 2d32340d28..24b69a1008 100644 > > > --- a/migration/savevm.c > > > +++ b/migration/savevm.c > > > @@ -2651,8 +2651,8 @@ retry: > > > while (true) { > > > section_type = qemu_get_byte(f); > > > > > > - if (qemu_file_get_error(f)) { > > > - ret = qemu_file_get_error(f); > > > + ret = qemu_file_get_error_obj_any(f, mis->postcopy_qemufile_dst, > > > NULL); > > > + if (ret) { > > > break; > > > } > > > ---8<--- > > > > > > Does it look sane? Let me know if there's still things missing. > > > > Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com> > > Thanks! > > -- > Peter Xu > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK