aio-posix: compute timeout before polling

This is a preparation for the next patch, and also a very small
optimization.  Compute the timeout only once, before invoking
try_poll_mode, and adjust it in run_poll_handlers.  The adjustment
is the polling time when polling fails, or zero (non-blocking) if
polling succeeds.

Fixes: 70232b5253
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180912171040.1732-3-pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

util/aio-posix.c

@@ -490,7 +490,7 @@ static void add_pollfd(AioHandler *node)
     npfd++;
 }
 
-static bool run_poll_handlers_once(AioContext *ctx)
+static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
 {
     bool progress = false;
     AioHandler *node;
@@ -500,6 +500,7 @@ static bool run_poll_handlers_once(AioContext *ctx)
             aio_node_check(ctx, node->is_external) &&
             node->io_poll(node->opaque) &&
             node->opaque != &ctx->notifier) {
+            *timeout = 0;
             progress = true;
         }
 
@@ -522,31 +523,38 @@ static bool run_poll_handlers_once(AioContext *ctx)
  *
  * Returns: true if progress was made, false otherwise
  */
-static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
+static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
 {
     bool progress;
-    int64_t end_time;
+    int64_t start_time, elapsed_time;
 
     assert(ctx->notify_me);
     assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
 
-    trace_run_poll_handlers_begin(ctx, max_ns);
-
-    end_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + max_ns;
+    trace_run_poll_handlers_begin(ctx, max_ns, *timeout);
 
+    start_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     do {
-        progress = run_poll_handlers_once(ctx);
-    } while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time
+        progress = run_poll_handlers_once(ctx, timeout);
+        elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
+    } while (!progress && elapsed_time < max_ns
              && !atomic_read(&ctx->poll_disable_cnt));
 
-    trace_run_poll_handlers_end(ctx, progress);
+    /* If time has passed with no successful polling, adjust *timeout to
+     * keep the same ending time.
+     */
+    if (*timeout != -1) {
+        *timeout -= MIN(*timeout, elapsed_time);
+    }
 
+    trace_run_poll_handlers_end(ctx, progress, *timeout);
     return progress;
 }
 
 /* try_poll_mode:
  * @ctx: the AioContext
- * @blocking: busy polling is only attempted when blocking is true
+ * @timeout: timeout for blocking wait, computed by the caller and updated if
+ *    polling succeeds.
  *
  * ctx->notify_me must be non-zero so this function can detect aio_notify().
  *
@@ -554,19 +562,16 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
  *
  * Returns: true if progress was made, false otherwise
  */
-static bool try_poll_mode(AioContext *ctx, bool blocking)
+static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
 {
-    if (blocking && ctx->poll_max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
-        /* See qemu_soonest_timeout() uint64_t hack */
-        int64_t max_ns = MIN((uint64_t)aio_compute_timeout(ctx),
-                             (uint64_t)ctx->poll_ns);
+    /* See qemu_soonest_timeout() uint64_t hack */
+    int64_t max_ns = MIN((uint64_t)*timeout, (uint64_t)ctx->poll_ns);
 
-        if (max_ns) {
-            poll_set_started(ctx, true);
+    if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
+        poll_set_started(ctx, true);
 
-            if (run_poll_handlers(ctx, max_ns)) {
-                return true;
-            }
+        if (run_poll_handlers(ctx, max_ns, timeout)) {
+            return true;
         }
     }
 
@@ -575,7 +580,7 @@ static bool try_poll_mode(AioContext *ctx, bool blocking)
     /* Even if we don't run busy polling, try polling once in case it can make
      * progress and the caller will be able to avoid ppoll(2)/epoll_wait(2).
      */
-    return run_poll_handlers_once(ctx);
+    return run_poll_handlers_once(ctx, timeout);
 }
 
 bool aio_poll(AioContext *ctx, bool blocking)
@@ -605,8 +610,14 @@ bool aio_poll(AioContext *ctx, bool blocking)
         start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     }
 
-    progress = try_poll_mode(ctx, blocking);
-    if (!progress) {
+    timeout = blocking ? aio_compute_timeout(ctx) : 0;
+    progress = try_poll_mode(ctx, &timeout);
+    assert(!(timeout && progress));
+
+    /* If polling is allowed, non-blocking aio_poll does not need the
+     * system call---a single round of run_poll_handlers_once suffices.
+     */
+    if (timeout || atomic_read(&ctx->poll_disable_cnt)) {
         assert(npfd == 0);
 
         /* fill pollfds */
@@ -620,8 +631,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
             }
         }
 
-        timeout = blocking ? aio_compute_timeout(ctx) : 0;
-
         /* wait until next event */
         if (aio_epoll_check_poll(ctx, pollfds, npfd, timeout)) {
             AioHandler epoll_handler;

util/trace-events

@@ -1,8 +1,8 @@
 # See docs/devel/tracing.txt for syntax documentation.
 
 # util/aio-posix.c
-run_poll_handlers_begin(void *ctx, int64_t max_ns) "ctx %p max_ns %"PRId64
-run_poll_handlers_end(void *ctx, bool progress) "ctx %p progress %d"
+run_poll_handlers_begin(void *ctx, int64_t max_ns, int64_t timeout) "ctx %p max_ns %"PRId64 " timeout %"PRId64
+run_poll_handlers_end(void *ctx, bool progress, int64_t timeout) "ctx %p progress %d new timeout %"PRId64
 poll_shrink(void *ctx, int64_t old, int64_t new) "ctx %p old %"PRId64" new %"PRId64
 poll_grow(void *ctx, int64_t old, int64_t new) "ctx %p old %"PRId64" new %"PRId64