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

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Date:   Thu, 1 Mar 2018 12:48:58 -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 Thu, Mar 01, 2018 at 12:04:04AM -0500, Steven Rostedt wrote:
> On Wed, 28 Feb 2018 17:21:44 -0800
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > > 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@...ux.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@...dmis.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.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);
> 
> The thing I hate about this is that it is invasive to code outside of
> the tracepoint benchmark. Why do the rcu_note_voluntary_context_switch
> here and not in the tracepoint code? Seems odd to have it called
> everywhere in the kernel when it is only needed by the benchmark
> tracepoint code.

Understood, and I am not completely devoid of sympathy for that view.
My problem with adding rcu_note_voluntary_context_switch() is that it
is a pretty deep detail of RCU.

Hmmm...  I wasn't happy with your original use of cond_resched_rcu_qs()
because it is now a rather strange thing.  However, this discussion has
helped me to understand that its real distinction over cond_resched()
as things stand now is that is provides a quiescent state for Tasks RCU.

So how about I rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which at least gives a hint as to where it needs to be used?

Would that work for you?

							Thanx, Paul

> -- Steve
> 
> 
> 
> > +	return 0;
> > +}
> >  #else
> >  static inline int _cond_resched(void) { return 0; }
> >  #endif
>