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

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Message-Id: <20180227153646.GD3777@linux.vnet.ibm.com>
Date:   Tue, 27 Feb 2018 07:36:46 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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 Mon, Feb 26, 2018 at 09:29:20PM -0500, Steven Rostedt wrote:
> 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.

Right, so I should use defined(CONFIG_TRACEPOINT_BENCHMARK) instead of
defined(CONFIG_TASKS_RCU).

Or am I misinterpreting the code in kernel/trace?

> > ------------------------------------------------------------------------
> > 
> > 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.

True, but based on the cond_resched_rcu_qs() experience, I bet that
trace_benchmark.c won't be the only place needing help.

							Thanx, Paul

> -- 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;
>