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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

For the set of 3:


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

* Peter Xu (peterx@redhat.com) wrote:
> 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

> From b3308e34398e21c19bd36ec21aae9c7f9f623d75 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@redhat.com>
> Date: Wed, 21 Sep 2022 09:51:55 -0400
> Subject: [PATCH] fixup! migration: Remove old preempt code around state
>  maintainance
> Content-type: text/plain
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/ram.c | 33 ---------------------------------
>  1 file changed, 33 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 03bf2324ab..2599eee070 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -97,28 +97,6 @@ struct PageSearchStatus {
>      unsigned long page;
>      /* Set once we wrap around */
>      bool         complete_round;
> -    /*
> -     * [POSTCOPY-ONLY] Whether current page is explicitly requested by
> -     * postcopy.  When set, the request is "urgent" because the dest QEMU
> -     * threads are waiting for us.
> -     */
> -    bool         postcopy_requested;
> -    /*
> -     * [POSTCOPY-ONLY] The target channel to use to send current page.
> -     *
> -     * Note: This may _not_ match with the value in postcopy_requested
> -     * above. Let's imagine the case where the postcopy request is exactly
> -     * the page that we're sending in progress during precopy. In this case
> -     * we'll have postcopy_requested set to true but the target channel
> -     * will be the precopy channel (so that we don't split brain on that
> -     * specific page since the precopy channel already contains partial of
> -     * that page data).
> -     *
> -     * Besides that specific use case, postcopy_target_channel should
> -     * always be equal to postcopy_requested, because by default we send
> -     * postcopy pages via postcopy preempt channel.
> -     */
> -    bool         postcopy_target_channel;
>      /* Whether we're sending a host page */
>      bool          host_page_sending;
>      /* The start/end of current host page.  Invalid if host_page_sending==false */
> @@ -1573,13 +1551,6 @@ retry:
>   */
>  static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss, bool *again)
>  {
> -    /*
> -     * This is not a postcopy requested page, mark it "not urgent", and use
> -     * precopy channel to send it.
> -     */
> -    pss->postcopy_requested = false;
> -    pss->postcopy_target_channel = RAM_CHANNEL_PRECOPY;
> -
>      /* Update pss->page for the next dirty bit in ramblock */
>      pss_find_next_dirty(pss);
>  
> @@ -2091,9 +2062,6 @@ static bool get_queued_page(RAMState *rs, PageSearchStatus *pss)
>           * really rare.
>           */
>          pss->complete_round = false;
> -        /* Mark it an urgent request, meanwhile using POSTCOPY channel */
> -        pss->postcopy_requested = true;
> -        pss->postcopy_target_channel = RAM_CHANNEL_POSTCOPY;
>      }
>  
>      return !!block;
> @@ -2190,7 +2158,6 @@ int ram_save_queue_pages(const char *rbname, ram_addr_t start, ram_addr_t len)
>           * we should be the only one who operates on the qemufile
>           */
>          pss->pss_channel = migrate_get_current()->postcopy_qemufile_src;
> -        pss->postcopy_requested = true;
>          assert(pss->pss_channel);
>  
>          /*
> -- 
> 2.32.0
> 

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK