[PATCH v3 net 1/3] rcu: add a helper to report consolidated flavor QS

[Yan Zhai]

There are several scenario in network processing that can run
extensively under heavy traffic. In such situation, RCU synchronization
might not observe desired quiescent states for indefinitely long period.
Create a helper to safely raise the desired RCU quiescent states for
such scenario.

Reviewed-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
---
 include/linux/rcupdate.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b663..e91ae38c33e3 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -247,6 +247,29 @@ do { \
 	cond_resched(); \
 } while (0)
 
+/**
+ * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
+ *
+ * This helper is for network processing in non-RT kernels, where there could
+ * be busy polling threads that block RCU synchronization indefinitely.  In
+ * such context, simply calling cond_resched is insufficient, so give it a
+ * stronger push to eliminate potential blockage of all RCU types.
+ *
+ * NOTE: unless absolutely sure, this helper should in general be called
+ * outside of bh lock section to avoid reporting a surprising QS to updaters,
+ * who could be expecting RCU read critical section to end at local_bh_enable().
+ */
+#define rcu_softirq_qs_periodic(old_ts) \
+do { \
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
+	    time_after(jiffies, (old_ts) + HZ / 10)) { \
+		preempt_disable(); \
+		rcu_softirq_qs(); \
+		preempt_enable(); \
+		(old_ts) = jiffies; \
+	} \
+} while (0)
+
 /*
  * Infrastructure to implement the synchronize_() primitives in
  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
-- 
2.30.2