[PATCH 4/6] service: tweak cycle statistics semantics

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

As a part of its service function, a service usually polls some kind
of source (e.g., an RX queue, a ring, an eventdev port, or a timer
wheel) to retrieve one or more items of work.

In low-load situations, the service framework reports a significant
amount of cycles spent for all running services, despite the fact they
have performed little or no actual work.

The per-call cycle expenditure for an idle service (i.e., a service
currently without pending jobs) is typically very low. Polling an
empty ring or RX queue is inexpensive. However, since the service
function call frequency on an idle or lightly loaded lcore is going to
be very high indeed, the service function calls' cycles adds up to a
significant amount. The only thing preventing the idle services'
cycles counters to make up 100% of the available CPU cycles is the
overhead of the service framework itself.

If the RTE_SERVICE_ATTR_CYCLES or RTE_SERVICE_LCORE_ATTR_CYCLES are
used to estimate service core load, the cores may look very busy when
the system is mostly doing nothing useful at all.

This patch allows for an idle service to indicate that no actual work
was performed during a particular service function call (by returning
-EAGAIN). In such cases the RTE_SERVICE_ATTR_CYCLES and
RTE_SERVICE_LCORE_ATTR_CYCLES values are not incremented.

The convention of returning -EAGAIN for idle services may in the
future also be used to have the lcore enter a short sleep, or reduce
its operating frequency, in case all services are currently idle.

This change is backward-compatible.

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
---
 lib/eal/common/rte_service.c            | 22 ++++++++++++++--------
 lib/eal/include/rte_service_component.h |  5 +++++
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/lib/eal/common/rte_service.c b/lib/eal/common/rte_service.c
index 4cac866792..123610688c 100644
--- a/lib/eal/common/rte_service.c
+++ b/lib/eal/common/rte_service.c
@@ -10,6 +10,7 @@
 #include <rte_service_component.h>
 
 #include <rte_lcore.h>
+#include <rte_branch_prediction.h>
 #include <rte_common.h>
 #include <rte_cycles.h>
 #include <rte_atomic.h>
@@ -364,24 +365,29 @@ service_runner_do_callback(struct rte_service_spec_impl *s,
 
 	if (service_stats_enabled(s)) {
 		uint64_t start = rte_rdtsc();
-		s->spec.callback(userdata);
-		uint64_t end = rte_rdtsc();
-		uint64_t cycles = end - start;
+		int rc = s->spec.callback(userdata);
 
 		/* The lcore service worker thread is the only writer,
 		 * and thus only a non-atomic load and an atomic store
 		 * is needed, and not the more expensive atomic
 		 * add.
 		 */
-		__atomic_store_n(&cs->cycles, cs->cycles + cycles,
-				 __ATOMIC_RELAXED);
+
+		if (likely(rc != -EAGAIN)) {
+			uint64_t end = rte_rdtsc();
+			uint64_t cycles = end - start;
+
+			__atomic_store_n(&cs->cycles, cs->cycles + cycles,
+					 __ATOMIC_RELAXED);
+			__atomic_store_n(&cs->cycles_per_service[service_idx],
+					 cs->cycles_per_service[service_idx] +
+					 cycles, __ATOMIC_RELAXED);
+		}
+
 		__atomic_store_n(&cs->calls_per_service[service_idx],
 				 cs->calls_per_service[service_idx] + 1,
 				 __ATOMIC_RELAXED);
 
-		__atomic_store_n(&cs->cycles_per_service[service_idx],
-				 cs->cycles_per_service[service_idx] + cycles,
-				 __ATOMIC_RELAXED);
 	} else
 		s->spec.callback(userdata);
 }
diff --git a/lib/eal/include/rte_service_component.h b/lib/eal/include/rte_service_component.h
index 9e66ee7e29..9be49d698a 100644
--- a/lib/eal/include/rte_service_component.h
+++ b/lib/eal/include/rte_service_component.h
@@ -19,6 +19,11 @@ extern "C" {
 
 /**
  * Signature of callback function to run a service.
+ *
+ * A service function call resulting in no actual work being
+ * performed, should return -EAGAIN. In that case, the (presumbly few)
+ * cycles spent will not be counted toward the lcore or service-level
+ * cycles attributes.
  */
 typedef int32_t (*rte_service_func)(void *args);
 
-- 
2.34.1