Re: [PATCH RFC v2 tip/core/rcu 09/22] rcu-tasks: Add an RCU-tasks rude variant

Paul E. McKenney
2020-03-19T23:58:32Z

On Thu, Mar 19, 2020 at 03:04:32PM -0400, Steven Rostedt wrote:
> On Wed, 18 Mar 2020 17:10:47 -0700
> paulmck@kernel.org wrote:
>
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> >
> > This commit adds a "rude" variant of RCU-tasks that has as quiescent
> > states schedule(), cond_resched_tasks_rcu_qs(), userspace execution,
> > and (in theory, anyway) cond_resched(). In other words, RCU-tasks rude
> > readers are regions of code with preemption disabled, but excluding code
> > early in the CPU-online sequence and late in the CPU-offline sequence.
> > Updates make use of IPIs and force an IPI and a context switch on each
> > online CPU. This variant is useful in some situations in tracing.
> >
> > Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> > [ paulmck: Apply EXPORT_SYMBOL_GPL() feedback from Qiujun Huang. ]
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> > include/linux/rcupdate.h | 3 ++
> > kernel/rcu/Kconfig | 12 +++++-
> > kernel/rcu/tasks.h | 98 ++++++++++++++++++++++++++++++++++++++++++++++++
> > 3 files changed, 112 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 5523145..2be97a8 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -37,6 +37,7 @@
> > /* Exported common interfaces */
> > void call_rcu(struct rcu_head *head, rcu_callback_t func);
> > void rcu_barrier_tasks(void);
> > +void rcu_barrier_tasks_rude(void);
> > void synchronize_rcu(void);
> >
> > #ifdef CONFIG_PREEMPT_RCU
> > @@ -138,6 +139,8 @@ static inline void rcu_init_nohz(void) { }
> > #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t)
> > void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
> > void synchronize_rcu_tasks(void);
> > +void call_rcu_tasks_rude(struct rcu_head *head, rcu_callback_t func);
> > +void synchronize_rcu_tasks_rude(void);
> > void exit_tasks_rcu_start(void);
> > void exit_tasks_rcu_finish(void);
> > #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
> > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > index 38475d0..0d43ec1 100644
> > --- a/kernel/rcu/Kconfig
> > +++ b/kernel/rcu/Kconfig
> > @@ -71,7 +71,7 @@ config TREE_SRCU
> > This option selects the full-fledged version of SRCU.
> >
> > config TASKS_RCU_GENERIC
> > - def_bool TASKS_RCU
> > + def_bool TASKS_RCU || TASKS_RUDE_RCU
> > select SRCU
> > help
> > This option enables generic infrastructure code supporting
> > @@ -84,6 +84,16 @@ config TASKS_RCU
> > only voluntary context switch (not preemption!), idle, and
> > user-mode execution as quiescent states. Not for manual selection.
> >
> > +config TASKS_RUDE_RCU
> > + def_bool 0
> > + default n
>
> No need for "default n" as that's the default without it.

Removed!

> > + help
> > + This option enables a task-based RCU implementation that uses
> > + only context switch (including preemption) and user-mode
> > + execution as quiescent states. It forces IPIs and context
> > + switches on all online CPUs, including idle ones, so use
> > + with caution. Not for manual selection.
>
> Really don't need the "Not for manual selection", as not having a prompt
> shows that too.

And also removed.

> > +
> > config RCU_STALL_COMMON
> > def_bool TREE_RCU
> > help
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index d77921e..7ba1730 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -180,6 +180,9 @@ static void __init rcu_tasks_bootup_oddness(void)
> > else
> > pr_info("\tTasks RCU enabled.\n");
> > #endif /* #ifdef CONFIG_TASKS_RCU */
> > +#ifdef CONFIG_TASKS_RUDE_RCU
> > + pr_info("\tRude variant of Tasks RCU enabled.\n");
> > +#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
> > }
> >
> > #endif /* #ifndef CONFIG_TINY_RCU */
> > @@ -410,3 +413,98 @@ static int __init rcu_spawn_tasks_kthread(void)
> > core_initcall(rcu_spawn_tasks_kthread);
> >
> > #endif /* #ifdef CONFIG_TASKS_RCU */
> > +
> > +#ifdef CONFIG_TASKS_RUDE_RCU
> > +
> > +////////////////////////////////////////////////////////////////////////
> > +//
> > +// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
> > +// passing an empty function to schedule_on_each_cpu(). This approach
> > +// provides an asynchronous call_rcu_rude() API and batching of concurrent
> > +// calls to the synchronous synchronize_rcu_rude() API. This sends IPIs
> > +// far and wide and induces otherwise unnecessary context switches on all
> > +// online CPUs, whether online or not.
>
> "on all online CPUs, whether online or not" ????

Good catch! It should be "whether idle or not". Fixed. ;-)

Thanx, Paul

> -- Steve
>
> > +
> > +// Empty function to allow workqueues to force a context switch.
> > +static void rcu_tasks_be_rude(struct work_struct *work)
> > +{
> > +}
> > +