Re: [PATCH 19/20] migration: Postcopy recover with preempt enabled

[Dr. David Alan Gilbert]

* Peter Xu (peterx@redhat.com) wrote:
> On Tue, Feb 22, 2022 at 11:32:10AM +0000, Dr. David Alan Gilbert wrote:
> > * Peter Xu (peterx@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 <dgilbert@redhat.com>

> 
> Thanks!
> 
> -- 
> Peter Xu
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK