[PULL,3/3] migration/dirtyrate: Fix precision losses and g_usleep overshoot

Commit Message

Yong Huang Aug. 28, 2023, 5:19 p.m. UTC

From: Andrei Gudkov <[e-mail]>

Signed-off-by: Andrei Gudkov <[e-mail]>
Reviewed-by: Hyman Huang <[e-mail]>
Message-Id: <8ddb0[e-mail]>
Signed-off-by: Hyman Huang <[e-mail]>
---
 migration/dirtyrate.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Patch

diff --git a/migration/dirtyrate.c b/migration/dirtyrate.c
index 84f1b0fb20..bccb3515e3 100644
--- a/migration/dirtyrate.c
+++ b/migration/dirtyrate.c
@@ -57,6 +57,8 @@  static int64_t dirty_stat_wait(int64_t msec, int64_t initial_time)
         msec = current_time - initial_time;
     } else {
         g_usleep((msec + initial_time - current_time) * 1000);
+        /* g_usleep may overshoot */
+        msec = qemu_clock_get_ms(QEMU_CLOCK_REALTIME) - initial_time;
     }
 
     return msec;
@@ -77,9 +79,13 @@  static int64_t do_calculate_dirtyrate(DirtyPageRecord dirty_pages,
 {
     uint64_t increased_dirty_pages =
         dirty_pages.end_pages - dirty_pages.start_pages;
-    uint64_t memory_size_MiB = qemu_target_pages_to_MiB(increased_dirty_pages);
 
-    return memory_size_MiB * 1000 / calc_time_ms;
+    /*
+     * multiply by 1000ms/s _before_ converting down to megabytes
+     * to avoid losing precision
+     */
+    return qemu_target_pages_to_MiB(increased_dirty_pages * 1000) /
+        calc_time_ms;
 }
 
 void global_dirty_log_change(unsigned int flag, bool start)