linux-kernel - Re: [PATCH tip/core/rcu 06/10] trace: Eliminate cond_resched_rcu_qs() in favor of cond

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Date:   Mon, 26 Feb 2018 21:29:20 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, 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, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH tip/core/rcu 06/10] trace: Eliminate
 cond_resched_rcu_qs() in favor of cond_resched()

On Sun, 25 Feb 2018 10:39:44 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > Hmmm...  Grasping at straws...  Could we make cond_resched() be something
> > like a tracepoint and instrument them with cond_resched_rcu_qs() if the
> > current RCU-tasks grace period ran for more that (say) a minute of its
> > ten-minute stall-warning span?  
> 
> On the other hand, you noted in your other email that the tracepoint
> benchmark should not be enabled on production systems.  So how about
> the following (again untested) patch?  The "defined(CONFIG_TASKS_RCU)"
> might need to change, especially if RCU-tasks is used in production
> kernels, but perhaps a starting point.

RCU tasks are used in production systems if PREEMPT is enabled (it
allows for optimizations with ftrace, perf and kprobes).

But the tracepoint is not used.

> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index b161ef8a902e..316c29c5e506 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1589,6 +1589,12 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
>   */
>  #ifndef CONFIG_PREEMPT
>  extern int _cond_resched(void);
> +#elif defined(CONFIG_TASKS_RCU)
> +static inline int _cond_resched(void)
> +{
> +	rcu_note_voluntary_context_switch(current);
> +	return 0;
> +}
>  #else
>  static inline int _cond_resched(void) { return 0; }
>  #endif


This does work, but so does the below, without causing cond_resched()
from being something other than a nop of CONFIG_PREEMPT.

-- Steve

diff --git a/kernel/trace/trace_benchmark.c b/kernel/trace/trace_benchmark.c
index 22fee766081b..82d83bb4874b 100644
--- a/kernel/trace/trace_benchmark.c
+++ b/kernel/trace/trace_benchmark.c
@@ -166,6 +166,7 @@ static int benchmark_event_kthread(void *arg)
 		 * block synchronize_rcu_tasks() indefinitely.
 		 */
 		cond_resched();
+		rcu_note_voluntary_context_switch(current);
 	}
 
 	return 0;