throttle: make throttle_config(throttle_get_config()) symmetric

Throttling has a weird property that throttle_get_config() does not
always return the same throttling settings that were given with
throttle_config().  In other words, the set and get functions aren't
symmetric.

If .max is 0 then the throttling code assigns a default value of .avg /
10 in throttle_config().  This is an implementation detail of the
throttling algorithm.  When throttle_get_config() is called the .max
value returned should still be 0.

Users are exposed to this quirk via "info block" or "query-block"
monitor commands.  This has caused confusion because it looks like a bug
when an unexpected value is reported.

This patch hides the .max value adjustment in throttle_get_config() and
updates test-throttle.c appropriately.

Reported-by: Nini Gu <ngu@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20170301115026.22621-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/util/throttle.c b/util/throttle.c
index 3817d9b..3570ed2 100644
--- a/util/throttle.c
+++ b/util/throttle.c
@@ -380,6 +380,14 @@ static void throttle_fix_bucket(LeakyBucket *bkt)
     }
 }
 
+/* undo internal bucket parameter changes (see throttle_fix_bucket()) */
+static void throttle_unfix_bucket(LeakyBucket *bkt)
+{
+    if (bkt->max < bkt->avg) {
+        bkt->max = 0;
+    }
+}
+
 /* take care of canceling a timer */
 static void throttle_cancel_timer(QEMUTimer *timer)
 {
@@ -420,7 +428,13 @@ void throttle_config(ThrottleState *ts,
  */
 void throttle_get_config(ThrottleState *ts, ThrottleConfig *cfg)
 {
+    int i;
+
     *cfg = ts->cfg;
+
+    for (i = 0; i < BUCKETS_COUNT; i++) {
+        throttle_unfix_bucket(&cfg->buckets[i]);
+    }
 }