Re: [dpdk-dev] [RFC PATCH v1 4/6] app/eventdev: add release barriers for pipeline test

[Feifei Wang <Feifei.Wang2@arm.com>]

Hi, Pavan

Sorry for my late reply and thanks very much for your review.

> -----Original Message-----
> From: Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>
> Sent: 2020年12月22日 18:33
> To: Feifei Wang <Feifei.Wang2@arm.com>; jerinj@marvell.com; Harry van
> Haaren <harry.van.haaren@intel.com>; Pavan Nikhilesh
> <pbhagavatula@caviumnetworks.com>
> Cc: dev@dpdk.org; nd <nd@arm.com>; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; stable@dpdk.org; Phil Yang
> <Phil.Yang@arm.com>
> Subject: RE: [RFC PATCH v1 4/6] app/eventdev: add release barriers for
> pipeline test
> 
> 
> >Add release barriers before updating the processed packets for worker
> >lcores to ensure the worker lcore has really finished data processing
> >and then it can update the processed packets number.
> >
> 
> I believe we can live with minor inaccuracies in stats being presented as
> atomics are pretty heavy when scheduler is limited to burst size as 1.
> 
> One option is to move it before a pipeline operation (pipeline_event_tx,
> pipeline_fwd_event etc.) as they imply implicit release barrier (as all the
> changes done to the event should be visible to the next core).

If I understand correctly, your meaning is that move release barriers before
pipeline_event_tx or pipeline_fwd_event. This can ensure the event has been
processed before the next core begins to tx/fwd. For example:

if (ev.sched_type == RTE_SCHED_TYPE_ATOMIC) {
		+	__atomic_thread_fence(__ATOMIC_RELEASE); 
			pipeline_event_tx(dev, port, &ev);
			w->processed_pkts++;
		} else {
			ev.queue_id++;
		+	__atomic_thread_fence(__ATOMIC_RELEASE);
			pipeline_fwd_event(&ev, RTE_SCHED_TYPE_ATOMIC);
			pipeline_event_enqueue(dev, port, &ev);

However, there are two reasons to prevent this:

First, compare with other tests in app/eventdev, for example, the eventdev perf test,
the wmb is after event operation to ensure operation has been finished and then w->processed_pkts++.
So, if we move release barriers before tx/fwd, it may cause that the tests of app/eventdev
become  inconsistent.This may reduce the maintainability of the code and make it difficult to understand.

Second, it is a test case, though heavy thread may cause performance degradation, it can ensure that
the operation process and the test result are correct. And maybe for a test case, correctness is more important
than performance.

So, due to two reasons above, I'm ambivalent about how we should do in the next step. 

Best Regards
Feifei

> >Fixes: 314bcf58ca8f ("app/eventdev: add pipeline queue worker
> >functions")
> >Cc: pbhagavatula@marvell.com
> >Cc: stable@dpdk.org
> >
> >Signed-off-by: Phil Yang <phil.yang@arm.com>
> >Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
> >Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> >---
> > app/test-eventdev/test_pipeline_queue.c | 64
> >+++++++++++++++++++++----
> > 1 file changed, 56 insertions(+), 8 deletions(-)
> >
> >diff --git a/app/test-eventdev/test_pipeline_queue.c b/app/test-
> >eventdev/test_pipeline_queue.c index 7bebac34f..0c0ec0ceb 100644
> >--- a/app/test-eventdev/test_pipeline_queue.c
> >+++ b/app/test-eventdev/test_pipeline_queue.c
> >@@ -30,7 +30,13 @@ pipeline_queue_worker_single_stage_tx(void
> >*arg)
> >
> > 		if (ev.sched_type == RTE_SCHED_TYPE_ATOMIC) {
> > 			pipeline_event_tx(dev, port, &ev);
> >-			w->processed_pkts++;
> >+
> >+			/* release barrier here ensures stored operation
> >+			 * of the event completes before the number of
> >+			 * processed pkts is visible to the main core
> >+			 */
> >+			__atomic_fetch_add(&(w->processed_pkts), 1,
> >+					__ATOMIC_RELEASE);
> > 		} else {
> > 			ev.queue_id++;
> > 			pipeline_fwd_event(&ev,
> >RTE_SCHED_TYPE_ATOMIC);
> >@@ -59,7 +65,13 @@ pipeline_queue_worker_single_stage_fwd(void
> >*arg)
> > 		rte_event_eth_tx_adapter_txq_set(ev.mbuf, 0);
> > 		pipeline_fwd_event(&ev, RTE_SCHED_TYPE_ATOMIC);
> > 		pipeline_event_enqueue(dev, port, &ev);
> >-		w->processed_pkts++;
> >+
> >+		/* release barrier here ensures stored operation
> >+		 * of the event completes before the number of
> >+		 * processed pkts is visible to the main core
> >+		 */
> >+		__atomic_fetch_add(&(w->processed_pkts), 1,
> >+				__ATOMIC_RELEASE);
> > 	}
> >
> > 	return 0;
> >@@ -84,7 +96,13 @@
> >pipeline_queue_worker_single_stage_burst_tx(void *arg)
> > 			if (ev[i].sched_type ==
> >RTE_SCHED_TYPE_ATOMIC) {
> > 				pipeline_event_tx(dev, port, &ev[i]);
> > 				ev[i].op = RTE_EVENT_OP_RELEASE;
> >-				w->processed_pkts++;
> >+
> >+				/* release barrier here ensures stored
> >operation
> >+				 * of the event completes before the
> >number of
> >+				 * processed pkts is visible to the main
> >core
> >+				 */
> >+				__atomic_fetch_add(&(w-
> >>processed_pkts), 1,
> >+						__ATOMIC_RELEASE);
> > 			} else {
> > 				ev[i].queue_id++;
> > 				pipeline_fwd_event(&ev[i],
> >@@ -121,7 +139,13 @@
> >pipeline_queue_worker_single_stage_burst_fwd(void *arg)
> > 		}
> >
> > 		pipeline_event_enqueue_burst(dev, port, ev, nb_rx);
> >-		w->processed_pkts += nb_rx;
> >+
> >+		/* release barrier here ensures stored operation
> >+		 * of the event completes before the number of
> >+		 * processed pkts is visible to the main core
> >+		 */
> >+		__atomic_fetch_add(&(w->processed_pkts), nb_rx,
> >+				__ATOMIC_RELEASE);
> > 	}
> >
> > 	return 0;
> >@@ -146,7 +170,13 @@ pipeline_queue_worker_multi_stage_tx(void
> >*arg)
> >
> > 		if (ev.queue_id == tx_queue[ev.mbuf->port]) {
> > 			pipeline_event_tx(dev, port, &ev);
> >-			w->processed_pkts++;
> >+
> >+			/* release barrier here ensures stored operation
> >+			 * of the event completes before the number of
> >+			 * processed pkts is visible to the main core
> >+			 */
> >+			__atomic_fetch_add(&(w->processed_pkts), 1,
> >+					__ATOMIC_RELEASE);
> > 			continue;
> > 		}
> >
> >@@ -180,7 +210,13 @@
> >pipeline_queue_worker_multi_stage_fwd(void *arg)
> > 			ev.queue_id = tx_queue[ev.mbuf->port];
> > 			rte_event_eth_tx_adapter_txq_set(ev.mbuf, 0);
> > 			pipeline_fwd_event(&ev,
> >RTE_SCHED_TYPE_ATOMIC);
> >-			w->processed_pkts++;
> >+
> >+			/* release barrier here ensures stored operation
> >+			 * of the event completes before the number of
> >+			 * processed pkts is visible to the main core
> >+			 */
> >+			__atomic_fetch_add(&(w->processed_pkts), 1,
> >+					__ATOMIC_RELEASE);
> > 		} else {
> > 			ev.queue_id++;
> > 			pipeline_fwd_event(&ev,
> >sched_type_list[cq_id]);
> >@@ -214,7 +250,13 @@
> >pipeline_queue_worker_multi_stage_burst_tx(void *arg)
> > 			if (ev[i].queue_id == tx_queue[ev[i].mbuf-
> >>port]) {
> > 				pipeline_event_tx(dev, port, &ev[i]);
> > 				ev[i].op = RTE_EVENT_OP_RELEASE;
> >-				w->processed_pkts++;
> >+
> >+				/* release barrier here ensures stored
> >operation
> >+				 * of the event completes before the
> >number of
> >+				 * processed pkts is visible to the main
> >core
> >+				 */
> >+				__atomic_fetch_add(&(w-
> >>processed_pkts), 1,
> >+						__ATOMIC_RELEASE);
> > 				continue;
> > 			}
> >
> >@@ -254,7 +296,13 @@
> >pipeline_queue_worker_multi_stage_burst_fwd(void *arg)
> >
> >	rte_event_eth_tx_adapter_txq_set(ev[i].mbuf, 0);
> > 				pipeline_fwd_event(&ev[i],
> >
> >	RTE_SCHED_TYPE_ATOMIC);
> >-				w->processed_pkts++;
> >+
> >+				/* release barrier here ensures stored
> >operation
> >+				 * of the event completes before the
> >number of
> >+				 * processed pkts is visible to the main
> >core
> >+				 */
> >+				__atomic_fetch_add(&(w-
> >>processed_pkts), 1,
> >+						__ATOMIC_RELEASE);
> > 			} else {
> > 				ev[i].queue_id++;
> > 				pipeline_fwd_event(&ev[i],
> >--
> >2.17.1