* 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.
> [...]
>
> > > @@ -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
