[PATCH v4 2/2] eal/unix: allow creating thread with real-time priority

[Thomas Monjalon <thomas@monjalon.net>]

When adding an API for creating threads,
the real-time priority has been forbidden on Unix.

There is a known issue with ring behaviour,
but it should not be completely forbidden.

Real-time thread can block some kernel threads on the same core,
making the system unstable.
That's why a pause is added in the test thread.

Fixes: ca04c78b6262 ("eal: get/set thread priority per thread identifier")
Fixes: ce6e911d20f6 ("eal: add thread lifetime API")
Fixes: a7ba40b2b1bf ("drivers: convert to internal control threads")
Cc: stable@dpdk.org

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
---
 app/test/test_threads.c                       | 11 +---------
 .../prog_guide/env_abstraction_layer.rst      |  4 +++-
 lib/eal/include/rte_thread.h                  |  8 +++++--
 lib/eal/unix/rte_thread.c                     | 21 +++++++++++--------
 4 files changed, 22 insertions(+), 22 deletions(-)

diff --git a/app/test/test_threads.c b/app/test/test_threads.c
index 4ac3f2671a..c14d39fc83 100644
--- a/app/test/test_threads.c
+++ b/app/test/test_threads.c
@@ -22,7 +22,7 @@ thread_main(void *arg)
 	__atomic_store_n(&thread_id_ready, 1, __ATOMIC_RELEASE);
 
 	while (__atomic_load_n(&thread_id_ready, __ATOMIC_ACQUIRE) == 1)
-		;
+		rte_thread_yield_realtime(); /* required for RT priority */
 
 	return 0;
 }
@@ -97,21 +97,12 @@ test_thread_priority(void)
 		"Priority set mismatches priority get");
 
 	priority = RTE_THREAD_PRIORITY_REALTIME_CRITICAL;
-#ifndef RTE_EXEC_ENV_WINDOWS
-	RTE_TEST_ASSERT(rte_thread_set_priority(thread_id, priority) == ENOTSUP,
-		"Priority set to critical should fail");
-	RTE_TEST_ASSERT(rte_thread_get_priority(thread_id, &priority) == 0,
-		"Failed to get thread priority");
-	RTE_TEST_ASSERT(priority == RTE_THREAD_PRIORITY_NORMAL,
-		"Failed set to critical should have retained normal");
-#else
 	RTE_TEST_ASSERT(rte_thread_set_priority(thread_id, priority) == 0,
 		"Priority set to critical should succeed");
 	RTE_TEST_ASSERT(rte_thread_get_priority(thread_id, &priority) == 0,
 		"Failed to get thread priority");
 	RTE_TEST_ASSERT(priority == RTE_THREAD_PRIORITY_REALTIME_CRITICAL,
 		"Priority set mismatches priority get");
-#endif
 
 	priority = RTE_THREAD_PRIORITY_NORMAL;
 	RTE_TEST_ASSERT(rte_thread_set_priority(thread_id, priority) == 0,
diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
index 6debf54efb..d1f7cae7cd 100644
--- a/doc/guides/prog_guide/env_abstraction_layer.rst
+++ b/doc/guides/prog_guide/env_abstraction_layer.rst
@@ -815,7 +815,9 @@ Known Issues
 
   4. It MAY be used by preemptible multi-producer and/or preemptible multi-consumer pthreads whose scheduling policy are all SCHED_OTHER(cfs), SCHED_IDLE or SCHED_BATCH. User SHOULD be aware of the performance penalty before using it.
 
-  5. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR.
+  5. It MUST not be used by multi-producer/consumer pthreads
+     whose scheduling policies are ``SCHED_FIFO``
+     or ``SCHED_RR`` (``RTE_THREAD_PRIORITY_REALTIME_CRITICAL``).
 
   Alternatively, applications can use the lock-free stack mempool handler. When
   considering this handler, note that:
diff --git a/lib/eal/include/rte_thread.h b/lib/eal/include/rte_thread.h
index f2581fe152..7ff031e1b2 100644
--- a/lib/eal/include/rte_thread.h
+++ b/lib/eal/include/rte_thread.h
@@ -56,10 +56,14 @@ typedef uint32_t (*rte_thread_func) (void *arg);
  * Thread priority values.
  */
 enum rte_thread_priority {
+	/** Normal thread priority, the default. */
 	RTE_THREAD_PRIORITY_NORMAL            = 0,
-	/**< normal thread priority, the default */
+	/**
+	 * Highest thread priority, use with caution.
+	 * WARNING: System may be unstable because of a real-time busy loop.
+	 *          @see rte_thread_yield_realtime().
+	 */
 	RTE_THREAD_PRIORITY_REALTIME_CRITICAL = 1,
-	/**< highest thread priority allowed */
 };
 
 /**
diff --git a/lib/eal/unix/rte_thread.c b/lib/eal/unix/rte_thread.c
index d0758f23bb..e9967e7e41 100644
--- a/lib/eal/unix/rte_thread.c
+++ b/lib/eal/unix/rte_thread.c
@@ -51,6 +51,18 @@ thread_map_priority_to_os_value(enum rte_thread_priority eal_pri, int *os_pri,
 			sched_get_priority_max(SCHED_OTHER)) / 2;
 		break;
 	case RTE_THREAD_PRIORITY_REALTIME_CRITICAL:
+		/*
+		 * WARNING: Real-time busy loop takes priority on kernel threads,
+		 *          making the system unstable.
+		 *          There is also a known issue when using rte_ring.
+		 */
+		static bool warned;
+		if (!warned) {
+			RTE_LOG(NOTICE, EAL,
+					"Real-time thread is unstable if polling without sleep.\n");
+			warned = true;
+		}
+
 		*pol = SCHED_RR;
 		*os_pri = sched_get_priority_max(SCHED_RR);
 		break;
@@ -155,11 +167,6 @@ rte_thread_create(rte_thread_t *thread_id,
 			goto cleanup;
 		}
 
-		if (thread_attr->priority ==
-				RTE_THREAD_PRIORITY_REALTIME_CRITICAL) {
-			ret = ENOTSUP;
-			goto cleanup;
-		}
 		ret = thread_map_priority_to_os_value(thread_attr->priority,
 				&param.sched_priority, &policy);
 		if (ret != 0)
@@ -292,10 +299,6 @@ rte_thread_set_priority(rte_thread_t thread_id,
 	int policy;
 	int ret;
 
-	/* Realtime priority can cause crashes on non-Windows platforms. */
-	if (priority == RTE_THREAD_PRIORITY_REALTIME_CRITICAL)
-		return ENOTSUP;
-
 	ret = thread_map_priority_to_os_value(priority, &param.sched_priority,
 		&policy);
 	if (ret != 0)
-- 
2.42.0