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 <[email protected]> Reviewed-by: Jesper Dangaard Brouer <[email protected]> Signed-off-by: Yan Zhai <[email protected]> --- 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
