migration: Wake rate limiting for urgent requests

Rate limiting sleeps the migration thread for a while when it runs
out of bandwidth; but sometimes we want to wake up to get on with
something more urgent (like a postcopy request).  Here we use
a semaphore with a timedwait instead of a simple sleep; Incrementing
the sempahore will wake it up sooner.  Anything that consumes
these urgent events must decrement the sempahore.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180613102642.23995-3-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/migration/migration.h b/migration/migration.h
index 31d3ed1..64a7b33 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -121,6 +121,11 @@ struct MigrationState
      */
     QemuMutex qemu_file_lock;
 
+    /*
+     * Used to allow urgent requests to override rate limiting.
+     */
+    QemuSemaphore rate_limit_sem;
+
     /* bytes already send at the beggining of current interation */
     uint64_t iteration_initial_bytes;
     /* time at the start of current iteration */
@@ -287,4 +292,7 @@ void init_dirty_bitmap_incoming_migration(void);
 #define qemu_ram_foreach_block \
   #warning "Use qemu_ram_foreach_block_migratable in migration code"
 
+void migration_make_urgent_request(void);
+void migration_consume_urgent_request(void);
+
 #endif