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: <20180228181252.55e0590c@vmware.local.home>
Date:   Wed, 28 Feb 2018 18:12:52 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 Tue, 27 Feb 2018 07:36:46 -0800
"Paul E. McKenney" <paulmck@...ux.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.

-- Steve