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SubjectRe: [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@linux.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;
>

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