[v3] rte_ring: clarify preemptible nature of ring algorithm

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  • [v3] rte_ring: clarify preemptible nature of ring algorithm
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Honnappa Nagarahalli July 10, 2018, 4 a.m.
rte_ring implementation is not preemptible only under certain
circumstances. This clarification is helpful for data plane and
control plane communication using rte_ring.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Reviewed-by: Gavin Hu <gavin.hu@arm.com>

Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>

---
v3:
* Corrected known issues for rte_ring
* Referred to known issues in rte_ring.h (Burakov, Oliver)

v2:
* Fixed checkpatch warnings

 doc/guides/prog_guide/env_abstraction_layer.rst | 27 ++++++++++++++-----------
 lib/librte_ring/rte_ring.h                      |  5 +++--
 2 files changed, 18 insertions(+), 14 deletions(-)

-- 
2.7.4

Patch

diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
index a22640d..f47a4be 100644
--- a/doc/guides/prog_guide/env_abstraction_layer.rst
+++ b/doc/guides/prog_guide/env_abstraction_layer.rst
@@ -435,23 +435,26 @@  Known Issues
 
     The "non-preemptive" constraint means:
 
-    - a pthread doing multi-producers enqueues on a given ring must not
-      be preempted by another pthread doing a multi-producer enqueue on
-      the same ring.
-    - a pthread doing multi-consumers dequeues on a given ring must not
-      be preempted by another pthread doing a multi-consumer dequeue on
-      the same ring.
+      A preempted pthread can block other pthreads (operating on the same ring)
+      from completing their operations, only if those pthreads are performing
+      the same ring operation (enqueue/dequeue) as the preempted pthread.
+      In other words, a preempted consumer pthread will not block any producer
+      pthreads and vice versa.
 
-    Bypassing this constraint may cause the 2nd pthread to spin until the 1st one is scheduled again.
-    Moreover, if the 1st pthread is preempted by a context that has an higher priority, it may even cause a dead lock.
+    Bypassing this constraint may cause other pthreads to spin until the preempted pthread is scheduled again.
+    Moreover, if the pthread is preempted by a context that has a higher priority, it may even cause a dead lock.
 
-  This does not mean it cannot be used, simply, there is a need to narrow down the situation when it is used by multi-pthread on the same core.
+  This means, use cases involving preemptible pthreads should consider using rte_ring carefully.
 
-  1. It CAN be used for any single-producer or single-consumer situation.
+  1. It CAN be used for preemptible single-producer and single-consumer use case.
 
-  2. It MAY be used by multi-producer/consumer pthread whose scheduling policy are all SCHED_OTHER(cfs). User SHOULD be aware of the performance penalty before using it.
+  2. It CAN be used for non-preemptible multi-producer and preemptible single-consumer use case.
 
-  3. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR.
+  3. It CAN be used for preemptible single-producer and non-preemptible multi-consumer use case.
+
+  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.
 
 + rte_timer
 
diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
index 1245822..e680101 100644
--- a/lib/librte_ring/rte_ring.h
+++ b/lib/librte_ring/rte_ring.h
@@ -26,8 +26,9 @@ 
  * - Bulk dequeue.
  * - Bulk enqueue.
  *
- * Note: the ring implementation is not preemptable. A lcore must not
- * be interrupted by another task that uses the same ring.
+ * Note: the ring implementation is not preemptible. Refer to Programmer's
+ * guide/Environment Abstraction Layer/Multiple pthread/Known Issues/rte_ring
+ * for more information.
  *
  */