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SubjectRe: [PATCH tip/core/rcu 06/10] trace: Eliminate cond_resched_rcu_qs() in favor of cond_resched()
On Wed, Feb 28, 2018 at 06:12:52PM -0500, Steven Rostedt wrote:
> On Tue, 27 Feb 2018 07:36:46 -0800
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > > > --- 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.
>
> Perhaps, still think this is a special case. That said, perhaps
> cond_resched isn't done in critical locations as it's a place that is
> explicitly stating that it's OK to schedule.

Building on your second sentence, when you are running a non-production
stress test, adding an extra function call and conditional branch to
cond_resched() should not be a problem.

So how about the (still untested) patch below?

Thanx, Paul

------------------------------------------------------------------------

commit e9a6ea9fc2542459f9a63cf2b3a0264d09fbc266
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Sun Feb 25 10:40:44 2018 -0800

EXP sched: Make non-production PREEMPT cond_resched() help Tasks RCU

In CONFIG_PREEMPT=y kernels, cond_resched() is a complete no-op, and
thus cannot help advance Tasks-RCU grace periods. However, such grace
periods are only an issue in non-production benchmarking runs of the
Linux kernel. This commit therefore makes cond_resched() invoke
rcu_note_voluntary_context_switch() for kernels implementing Tasks RCU
even in CONFIG_PREEMPT=y kernels.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b161ef8a902e..970dadefb86f 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_TRACEPOINT_BENCHMARK)
+static inline int _cond_resched(void)
+{
+ rcu_note_voluntary_context_switch(current);
+ return 0;
+}
#else
static inline int _cond_resched(void) { return 0; }
#endif
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