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

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Date:   Thu, 19 Mar 2020 15:04:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org
Subject: Re: [PATCH RFC v2 tip/core/rcu 09/22] rcu-tasks: Add an RCU-tasks
 rude variant

On Wed, 18 Mar 2020 17:10:47 -0700
paulmck@...nel.org wrote:

> From: "Paul E. McKenney" <paulmck@...nel.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@...dmis.org>
> [ paulmck: Apply EXPORT_SYMBOL_GPL() feedback from Qiujun Huang. ]
> Signed-off-by: Paul E. McKenney <paulmck@...nel.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.

> +	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.

> +
>  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" ????

-- Steve

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