linux-kernel - Re: [PATCH tip/core/rcu 01/22] sched: Make non-production PREEMPT cond

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Date:   Mon, 23 Apr 2018 05:40:00 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.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, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel.opensrc@...il.com
Subject: Re: [PATCH tip/core/rcu 01/22] sched: Make non-production PREEMPT
 cond_resched() help Tasks RCU

On Mon, Apr 23, 2018 at 10:51:27AM +0200, Peter Zijlstra wrote:
> On Sun, Apr 22, 2018 at 07:32:06PM -0700, Paul E. McKenney wrote:
> > 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.
> 
> I'm confused.. why is having this conditional on TRACEPOINT_BENCHMARK a
> sane idea?

Because the TRACEPOINT_BENCHMARK tests are insane, so a similar
level of insanity is required to make things work.  Plus having this
be unconditional would not be good for performance, as 0day has been
telling me frequently over the past couple of years.

All that aside, I am very open to ideas.  What would you suggest?

							Thanx, Paul

> > Reported-by: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  include/linux/sched.h | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index b3d697f3b573..fe8f7178a22d 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1617,6 +1617,12 @@ 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
> > -- 
> > 2.5.2
> > 
>