On 30/09/21 00:10, Frederic Weisbecker wrote: > On PREEMPT_RT, if rcu_core() is preempted by the de-offloading process, > some work, such as callbacks acceleration and invocation, may be left > unattended due to the volatile checks on the offloaded state. > > In the worst case this work is postponed until the next rcu_pending() > check that can take a jiffy to reach, which can be a problem in case > of callbacks flooding. > > Solve that with invoking rcu_core() early in the de-offloading process. > This way any work dismissed by an ongoing rcu_core() call fooled by > a preempting deoffloading process will be caught up by a nearby future > recall to rcu_core(), this time fully aware of the de-offloading state. > > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> > Cc: Valentin Schneider <valentin.schneider@xxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx> > Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> > Cc: Uladzislau Rezki <urezki@xxxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> One comment/question below. > @@ -990,6 +990,15 @@ static long rcu_nocb_rdp_deoffload(void *arg) > * will refuse to put anything into the bypass. > */ > WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies)); > + /* > + * Start with invoking rcu_core() early. This way if the current thread > + * happens to preempt an ongoing call to rcu_core() in the middle, > + * leaving some work dismissed because rcu_core() still thinks the rdp is > + * completely offloaded, we are guaranteed a nearby future instance of > + * rcu_core() to catch up. > + */ > + rcu_segcblist_set_flags(cblist, SEGCBLIST_RCU_CORE); > + invoke_rcu_core(); I think your approach is a bit neater, but would there have been any issue with keeping the setting of SEGCBLIST_RCU_CORE within rcu_segcblist_offload() and bundling it with an invoke_rcu_core()? > ret = rdp_offload_toggle(rdp, false, flags); > swait_event_exclusive(rdp->nocb_state_wq, > !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB | > -- > 2.25.1
