Re: [PATCH 13/14] migration: Remove old preempt code around state maintainance

[Peter Xu]

On Tue, Sep 20, 2022 at 08:47:20PM -0400, Peter Xu wrote:
> On Tue, Sep 20, 2022 at 06:52:27PM -0400, Peter Xu wrote:
> > With the new code to send pages in rp-return thread, there's little help to
> > keep lots of the old code on maintaining the preempt state in migration
> > thread, because the new way should always be faster..
> > 
> > Then if we'll always send pages in the rp-return thread anyway, we don't
> > need those logic to maintain preempt state anymore because now we serialize
> > things using the mutex directly instead of using those fields.
> > 
> > It's very unfortunate to have those code for a short period, but that's
> > still one intermediate step that we noticed the next bottleneck on the
> > migration thread.  Now what we can do best is to drop unnecessary code as
> > long as the new code is stable to reduce the burden.  It's actually a good
> > thing because the new "sending page in rp-return thread" model is (IMHO)
> > even cleaner and with better performance.
> > 
> > Remove the old code that was responsible for maintaining preempt states, at
> > the meantime also remove x-postcopy-preempt-break-huge parameter because
> > with concurrent sender threads we don't really need to break-huge anymore.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/migration.c |   2 -
> >  migration/ram.c       | 258 +-----------------------------------------
> >  2 files changed, 3 insertions(+), 257 deletions(-)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index fae8fd378b..698fd94591 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -4399,8 +4399,6 @@ static Property migration_properties[] = {
> >      DEFINE_PROP_SIZE("announce-step", MigrationState,
> >                        parameters.announce_step,
> >                        DEFAULT_MIGRATE_ANNOUNCE_STEP),
> > -    DEFINE_PROP_BOOL("x-postcopy-preempt-break-huge", MigrationState,
> > -                      postcopy_preempt_break_huge, true),
> 
> Forgot to drop the variable altogether:
> 
> diff --git a/migration/migration.h b/migration/migration.h
> index cdad8aceaa..ae4ffd3454 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -340,13 +340,6 @@ struct MigrationState {
>      bool send_configuration;
>      /* Whether we send section footer during migration */
>      bool send_section_footer;
> -    /*
> -     * Whether we allow break sending huge pages when postcopy preempt is
> -     * enabled.  When disabled, we won't interrupt precopy within sending a
> -     * host huge page, which is the old behavior of vanilla postcopy.
> -     * NOTE: this parameter is ignored if postcopy preempt is not enabled.
> -     */
> -    bool postcopy_preempt_break_huge;
>  
>      /* Needed by postcopy-pause state */
>      QemuSemaphore postcopy_pause_sem;
> 
> Will squash this in in next version.

Two more varialbes to drop, as attached..


-- 
Peter Xu

0001-fixup-migration-Remove-old-preempt-code-around-state.patch
Description: Text document