On Wed, Mar 20, 2019 at 12:28:35PM +0100, Sebastian Andrzej Siewior wrote: > On 2019-03-19 20:26:13 [-0400], Joel Fernandes wrote: > > > @@ -2769,19 +2782,121 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp) > > > { > > > if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > > > return; > > > - if (likely(!rcu_state.boost)) { > > > - rcu_do_batch(rdp); > > > - return; > > > - } > > > - invoke_rcu_callbacks_kthread(); > > > + rcu_do_batch(rdp); > > > > Looks like a nice change, but one question... > > > > Consider the case where rcunosoftirq boot option is not passed. > > > > Before, if RCU_BOOST=y, then callbacks would be invoked in rcuc threads if > > possible, by those threads being woken up from within the softirq context > > (in invoke_rcu_callbacks). > > > > Now, if RCU_BOOST=y, then callbacks would only be invoked in softirq context > > and not in the threads at all. Because rcu_softirq_enabled = false, so the > > path executes: > > rcu_read_unlock_special() -> > > raise_softirq_irqsoff() -> > > rcu_process_callbacks_si() -> > > rcu_process_callbacks() -> > > invoke_rcu_callbacks() -> > > rcu_do_batch() > > > > This seems like a behavioral change to me. This makes the callbacks always > > execute from the softirq context and not the threads when boosting is > > configured. IMO in the very least, such behavioral change should be > > documented in the change. > > > > One way to fix this I think could be, if boosting is enabled, then set > > rcu_softirq_enabled to false by default so the callbacks are still executed > > in the rcuc threads. > > > > Did I miss something? Sorry if I did, thanks! > > So with all the swaps and reorder we talking about this change: > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 0a719f726e149..82810483bfc6c 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2306,20 +2306,6 @@ static void rcu_core_si(struct softirq_action *h) > rcu_core(); > } > > -/* > - * Schedule RCU callback invocation. If the running implementation of RCU > - * does not support RCU priority boosting, just do a direct call, otherwise > - * wake up the per-CPU kernel kthread. Note that because we are running > - * on the current CPU with softirqs disabled, the rcu_cpu_kthread_task > - * cannot disappear out from under us. > - */ > -static void invoke_rcu_callbacks(struct rcu_data *rdp) > -{ > - if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > - return; > - rcu_do_batch(rdp); > -} > - > static void rcu_wake_cond(struct task_struct *t, int status) > { > /* > @@ -2330,6 +2316,19 @@ static void rcu_wake_cond(struct task_struct *t, int status) > wake_up_process(t); > } > > +static void invoke_rcu_core_kthread(void) > +{ > + struct task_struct *t; > + unsigned long flags; > + > + local_irq_save(flags); > + __this_cpu_write(rcu_data.rcu_cpu_has_work, 1); > + t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task); > + if (t != NULL && t != current) > + rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status)); > + local_irq_restore(flags); > +} > + > static bool rcu_softirq_enabled = true; > > static int __init rcunosoftirq_setup(char *str) > @@ -2339,26 +2338,33 @@ static int __init rcunosoftirq_setup(char *str) > } > __setup("rcunosoftirq", rcunosoftirq_setup); > > +/* > + * Schedule RCU callback invocation. If the running implementation of RCU > + * does not support RCU priority boosting, just do a direct call, otherwise > + * wake up the per-CPU kernel kthread. Note that because we are running > + * on the current CPU with softirqs disabled, the rcu_cpu_kthread_task > + * cannot disappear out from under us. > + */ > +static void invoke_rcu_callbacks(struct rcu_data *rdp) > +{ > + if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > + return; > + if (rcu_state.boost || rcu_softirq_enabled) > + invoke_rcu_core_kthread(); Here shouldn't it be this? if (rcu_state.boost || !rcu_softirq_enabled) Also the rcu/dev branch has the following hunk where we unconditionally invoke rcu_do_batch even when boosting which would still have the issue I pointed. I would suggest Sebastian to post the latest v4 or v5 with all diff squashed, and then we do another round of review with latest patch, thanks! @@ -2306,18 +2320,110 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp) { if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) return; - if (likely(!rcu_state.boost)) { - rcu_do_batch(rdp); - return; - } - invoke_rcu_callbacks_kthread(); + rcu_do_batch(rdp); +} + thanks, - Joel > + rcu_do_batch(rdp); > +} > + > /* > * Wake up this CPU's rcuc kthread to do RCU core processing. > */ > static void invoke_rcu_core(void) > { > - unsigned long flags; > - struct task_struct *t; > - > if (!cpu_online(smp_processor_id())) > return; > - if (rcu_softirq_enabled) { > + if (rcu_softirq_enabled) > raise_softirq(RCU_SOFTIRQ); > - } else { > - local_irq_save(flags); > - __this_cpu_write(rcu_data.rcu_cpu_has_work, 1); > - t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task); > - if (t != NULL && t != current) > - rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status)); > - local_irq_restore(flags); > - } > + else > + invoke_rcu_core_kthread(); > } > > static void rcu_cpu_kthread_park(unsigned int cpu) > @@ -2426,7 +2432,8 @@ static int __init rcu_spawn_core_kthreads(void) > per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0; > if (!IS_ENABLED(CONFIG_RCU_BOOST) && !rcu_softirq_enabled) > return 0; > - WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__); > + WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), > + "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__); > return 0; > } > early_initcall(rcu_spawn_core_kthreads); > -- > 2.20.1 > > > - Joel > > Sebastian