ratelimit: don't align wait time with slices

It is possible for rate limited writes to keep overshooting a slice's
quota by a tiny amount causing the slice-aligned waiting period to
effectively halve the rate.
Signed-off-by: NWolfgang Bumiller <w.bumiller@proxmox.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Message-id: 20180207071758.6818-1-w.bumiller@proxmox.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

include/qemu/ratelimit.h

@@ -36,7 +36,7 @@ typedef struct {
 static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
 {
     int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
-    uint64_t delay_slices;
+    double delay_slices;
 
     assert(limit->slice_quota && limit->slice_ns);
 
@@ -55,12 +55,11 @@ static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
         return 0;
     }
 
-    /* Quota exceeded. Calculate the next time slice we may start
-     * sending data again. */
-    delay_slices = (limit->dispatched + limit->slice_quota - 1) /
-        limit->slice_quota;
+    /* Quota exceeded. Wait based on the excess amount and then start a new
+     * slice. */
+    delay_slices = (double)limit->dispatched / limit->slice_quota;
     limit->slice_end_time = limit->slice_start_time +
-        delay_slices * limit->slice_ns;
+        (uint64_t)(delay_slices * limit->slice_ns);
     return limit->slice_end_time - now;
 }