[dpdk-dev] [PATCH v4 1/2] service: add API to retrieve service core active

Harry van Haaren harry.van.haaren at intel.com
Fri Jul 24 14:45:02 CEST 2020


This commit adds a new experimental API which allows the user
to retrieve the active state of an lcore. Knowing when the service
lcore is completed its polling loop can be useful to applications
to avoid race conditions when e.g. finalizing statistics.

The service thread itself now has a variable to indicate if its
thread is active. When zero the service thread has completed its
service, and has returned from the service_runner_func() function.

Suggested-by: Lukasz Wojciechowski <l.wojciechow at partner.samsung.com>
Signed-off-by: Harry van Haaren <harry.van.haaren at intel.com>
Reviewed-by: Phil Yang <phil.yang at arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>

---

v4:
- Use _may_be_ style API for lcore_active (Honnappa)
- Fix missing tab indent (Honnappa)
- Add 'lcore' to doxygen retval description (Honnappa)

@Honnappa: Please note i did not update the doxygen title of the
lcore_may_be_active() function, as the current description is more
accurate than making it more consistent with other functions. The
function returns core active state - and I don't want to confuse
mappings of services to lcores with what the function returns.

v3:
- Change service lcore stores to SEQ_CST (Honnappa, David)
- Change control thread load to ACQ (Honnappa, David)
- Comment reasons for SEQ_CST/ACQ (Honnappa, David)
- Add comments to Doxygen for _stop() and _lcore_active() (Honnappa, David)
- Add Phil's review tag from ML
---
 lib/librte_eal/common/rte_service.c  | 21 +++++++++++++++++++++
 lib/librte_eal/include/rte_service.h | 22 +++++++++++++++++++++-
 lib/librte_eal/rte_eal_version.map   |  1 +
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/lib/librte_eal/common/rte_service.c b/lib/librte_eal/common/rte_service.c
index 6a0e0ff65..35f1887fd 100644
--- a/lib/librte_eal/common/rte_service.c
+++ b/lib/librte_eal/common/rte_service.c
@@ -65,6 +65,7 @@ struct core_state {
 	/* map of services IDs are run on this core */
 	uint64_t service_mask;
 	uint8_t runstate; /* running or stopped */
+	uint8_t thread_active; /* indicates when thread is in service_run() */
 	uint8_t is_service_core; /* set if core is currently a service core */
 	uint8_t service_active_on_lcore[RTE_SERVICE_NUM_MAX];
 	uint64_t loops;
@@ -457,6 +458,8 @@ service_runner_func(void *arg)
 	const int lcore = rte_lcore_id();
 	struct core_state *cs = &lcore_states[lcore];
 
+	__atomic_store_n(&cs->thread_active, 1, __ATOMIC_SEQ_CST);
+
 	/* runstate act as the guard variable. Use load-acquire
 	 * memory order here to synchronize with store-release
 	 * in runstate update functions.
@@ -475,9 +478,27 @@ service_runner_func(void *arg)
 		cs->loops++;
 	}
 
+	/* Use SEQ CST memory ordering to avoid any re-ordering around
+	 * this store, ensuring that once this store is visible, the service
+	 * lcore thread really is done in service cores code.
+	 */
+	__atomic_store_n(&cs->thread_active, 0, __ATOMIC_SEQ_CST);
 	return 0;
 }
 
+int32_t
+rte_service_lcore_may_be_active(uint32_t lcore)
+{
+	if (lcore >= RTE_MAX_LCORE || !lcore_states[lcore].is_service_core)
+		return -EINVAL;
+
+	/* Load thread_active using ACQUIRE to avoid instructions dependant on
+	 * the result being re-ordered before this load completes.
+	 */
+	return __atomic_load_n(&lcore_states[lcore].thread_active,
+			       __ATOMIC_ACQUIRE);
+}
+
 int32_t
 rte_service_lcore_count(void)
 {
diff --git a/lib/librte_eal/include/rte_service.h b/lib/librte_eal/include/rte_service.h
index e2d0a6dd3..09f0d08b1 100644
--- a/lib/librte_eal/include/rte_service.h
+++ b/lib/librte_eal/include/rte_service.h
@@ -249,7 +249,11 @@ int32_t rte_service_lcore_start(uint32_t lcore_id);
  * Stop a service core.
  *
  * Stopping a core makes the core become idle, but remains  assigned as a
- * service core.
+ * service core. Note that the serivce lcore thread may not have returned from
+ * the service it is running when this API returns.
+ *
+ * The *rte_service_lcore_may_be_active* API can be used to check if the
+ * service lcore is * still active.
  *
  * @retval 0 Success
  * @retval -EINVAL Invalid *lcore_id* provided
@@ -261,6 +265,22 @@ int32_t rte_service_lcore_start(uint32_t lcore_id);
  */
 int32_t rte_service_lcore_stop(uint32_t lcore_id);
 
+/**
+ * Reports if a service lcore is currently running.
+ *
+ * This function returns if the core has finished service cores code, and has
+ * returned to EAL control. If *rte_service_lcore_stop* has been called but
+ * the lcore has not returned to EAL yet, it might be required to wait and call
+ * this function again. The amount of time to wait before the core returns
+ * depends on the duration of the services being run.
+ *
+ * @retval 0 Service thread is not active, and lcore has been returned to EAL.
+ * @retval 1 Service thread is in the service core polling loop.
+ * @retval -EINVAL Invalid *lcore_id* provided.
+ */
+__rte_experimental
+int32_t rte_service_lcore_may_be_active(uint32_t lcore_id);
+
 /**
  * Adds lcore to the list of service cores.
  *
diff --git a/lib/librte_eal/rte_eal_version.map b/lib/librte_eal/rte_eal_version.map
index bf0c17c23..39826ef91 100644
--- a/lib/librte_eal/rte_eal_version.map
+++ b/lib/librte_eal/rte_eal_version.map
@@ -401,6 +401,7 @@ EXPERIMENTAL {
 	rte_lcore_dump;
 	rte_lcore_iterate;
 	rte_mp_disable;
+	rte_service_lcore_may_be_active;
 	rte_thread_register;
 	rte_thread_unregister;
 };
-- 
2.17.1



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