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 16:58:29 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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 Thu, Mar 19, 2020 at 03:04:32PM -0400, Steven Rostedt wrote:
> 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.

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)
> > +{
> > +}
> > +