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.
Currently the frequency is locked at HZ/10, i.e. 100ms, which is
sufficient to address existing problems around RCU tasks. It's unclear
yet if there is any future scenario for it to be further tuned down.
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
---
v3->v4: comment fixup
---
include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b663..da224706323e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -247,6 +247,30 @@ do { \
cond_resched(); \
} while (0)
+/**
+ * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
+ * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
+ *
+ * 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 all 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
