[PATCH v2 3/5] throttle: support read-only and write-only

[zhenwei pi]

Only one direction is necessary in several scenarios:
- a read-only disk
- operations on a device are considered as *write* only. For example,
  encrypt/decrypt/sign/verify operations on a cryptodev use a single
  *write* timer(read timer callback is defined, but never invoked).

Allow a single direction in throttle, this reduces memory, and uplayer
does not need a dummy callback any more.

Signed-off-by: zhenwei pi <pizhen...@bytedance.com>
---
 util/throttle.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/util/throttle.c b/util/throttle.c
index 5642e61763..c0bd0c26c3 100644
--- a/util/throttle.c
+++ b/util/throttle.c
@@ -199,12 +199,17 @@ static bool throttle_compute_timer(ThrottleState *ts,
 void throttle_timers_attach_aio_context(ThrottleTimers *tt,
                                         AioContext *new_context)
 {
-    tt->timers[THROTTLE_READ] =
-        aio_timer_new(new_context, tt->clock_type, SCALE_NS,
-                      tt->timer_cb[THROTTLE_READ], tt->timer_opaque);
-    tt->timers[THROTTLE_WRITE] =
-        aio_timer_new(new_context, tt->clock_type, SCALE_NS,
-                      tt->timer_cb[THROTTLE_WRITE], tt->timer_opaque);
+    if (tt->timer_cb[THROTTLE_READ]) {
+        tt->timers[THROTTLE_READ] =
+            aio_timer_new(new_context, tt->clock_type, SCALE_NS,
+                          tt->timer_cb[THROTTLE_READ], tt->timer_opaque);
+    }
+
+    if (tt->timer_cb[THROTTLE_WRITE]) {
+        tt->timers[THROTTLE_WRITE] =
+            aio_timer_new(new_context, tt->clock_type, SCALE_NS,
+                          tt->timer_cb[THROTTLE_WRITE], tt->timer_opaque);
+    }
 }
 
 /*
@@ -235,6 +240,7 @@ void throttle_timers_init(ThrottleTimers *tt,
                           QEMUTimerCB *write_timer_cb,
                           void *timer_opaque)
 {
+    assert(read_timer_cb || write_timer_cb);
     memset(tt, 0, sizeof(ThrottleTimers));
 
     tt->clock_type = clock_type;
@@ -247,7 +253,9 @@ void throttle_timers_init(ThrottleTimers *tt,
 /* destroy a timer */
 static void throttle_timer_destroy(QEMUTimer **timer)
 {
-    assert(*timer != NULL);
+    if (*timer == NULL) {
+        return;
+    }
 
     timer_free(*timer);
     *timer = NULL;
@@ -272,7 +280,7 @@ void throttle_timers_destroy(ThrottleTimers *tt)
 /* is any throttling timer configured */
 bool throttle_timers_are_initialized(ThrottleTimers *tt)
 {
-    if (tt->timers[0]) {
+    if (tt->timers[THROTTLE_READ] || tt->timers[THROTTLE_WRITE]) {
         return true;
     }
 
@@ -424,8 +432,12 @@ bool throttle_schedule_timer(ThrottleState *ts,
 {
     int64_t now = qemu_clock_get_ns(tt->clock_type);
     int64_t next_timestamp;
+    QEMUTimer *timer;
     bool must_wait;
 
+    timer = is_write ? tt->timers[THROTTLE_WRITE] : tt->timers[THROTTLE_READ];
+    assert(timer);
+
     must_wait = throttle_compute_timer(ts,
                                        is_write,
                                        now,
@@ -437,12 +449,12 @@ bool throttle_schedule_timer(ThrottleState *ts,
     }
 
     /* request throttled and timer pending -> do nothing */
-    if (timer_pending(tt->timers[is_write])) {
+    if (timer_pending(timer)) {
         return true;
     }
 
     /* request throttled and timer not pending -> arm timer */
-    timer_mod(tt->timers[is_write], next_timestamp);
+    timer_mod(timer, next_timestamp);
     return true;
 }
 
-- 
2.34.1