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SubjectRe: [PATCH] rcu: Allow to eliminate softirq processing from rcutree
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

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