thread-pool: optimize scheduling of completion bottom half

The completion bottom half was scheduled within the pool->lock
critical section.  That actually results in worse performance,
because the worker thread can run its own small critical section
and go to sleep before the bottom half starts running.

Note that this simple change does not produce an improvement without
changing the thread pool QemuSemaphore to a condition variable.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Message-Id: <20220514065012.1149539-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/thread-pool.c

@@ -127,9 +127,8 @@ static void *worker_thread(void *opaque)
         smp_wmb();
         req->state = THREAD_DONE;
 
-        qemu_mutex_lock(&pool->lock);
-
         qemu_bh_schedule(pool->completion_bh);
+        qemu_mutex_lock(&pool->lock);
     }
 
     pool->cur_threads--;