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

[Peter Xu <peterx@redhat.com>]

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.

-- 
Peter Xu