On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote: > 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.
I suggest something like the following for the commit log: ------------------------------------------------------------------------ When under heavy load, network processing can run CPU-bound for many tens of seconds. Even in preemptible kernels, this can block RCU Tasks grace periods, which can cause trace-event removal to take more than a minute, which is unacceptably long. This commit therefore creates a new helper function that passes through both RCU and RCU-Tasks quiescent states every 100 milliseconds. This hard-coded value suffices for current workloads. ------------------------------------------------------------------------ > 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(). > + */ How about something like this for the kernel-doc comment? /** * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states * @old_ts: jiffies at start of processing. * * This helper is for long-running softirq handlers, such as those * in networking. The caller should initialize the variable passed in * as @old_ts at the beginning of the softirq handler. When invoked * frequently, this macro will invoke rcu_softirq_qs() every 100 * milliseconds thereafter, which will provide both RCU and RCU-Tasks * quiescent states. Note that this macro modifies its old_ts argument. * * Note that although cond_resched() provides RCU quiescent states, * it does not provide RCU-Tasks quiescent states. * * Because regions of code that have disabled softirq act as RCU * read-side critical sections, this macro should be invoked with softirq * (and preemption) enabled. * * This macro has no effect in CONFIG_PREEMPT_RT kernels. */ Thanx, Paul > +#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 > >
