On 2020-11-24 01:04, Jakub Kicinski wrote:

On Thu, 19 Nov 2020 09:30:15 +0100 Björn Töpel wrote:

quoted

+	/* The NAPI context has more processing work, but busy-polling
+	 * is preferred. Exit early.
+	 */
+	if (napi_prefer_busy_poll(n)) {
+		if (napi_complete_done(n, work)) {
+			/* If timeout is not set, we need to make sure
+			 * that the NAPI is re-scheduled.
+			 */
+			napi_schedule(n);
+		}
+		goto out_unlock;
+	}

Do we really need to go through napi_complete_done() here?

Isn't it sufficient to check:

	if (napi_prefer_busy_poll(n) &&
	    hrtimer_active(&n->timer)) // not 100% sure this is the
	                               // right helper for the check

If timer is scheduled it will fire and worst case sirq will kick back
in after timeout. napi_complete_done() should had been called by the
driver already to schedule the timer. If the driver doesn't call
napi_complete_done() we should not allow it to use busy_poll() anyway.

No, it's not. For a heavy traffic load, the napi_complete_done() will
never be called by the driver. It'll just keep on spinning in the
ksoftirqd. This code is to force out of that loop, so we need to call
napi_complete_done() explicitly (which will set the timeout).

Without the explicit napi_complete_done(), the ksoftirqd will not stop,
and the busy-polling will never allow to enter.


Björn