linux-kernel - Re: [PATCH tip/core/rcu 10/10] rcu: Account for rcu_all_qs() in cond

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Message-Id: <20171202122220.GM7829@linux.vnet.ibm.com>
Date:   Sat, 2 Dec 2017 04:22:20 -0800
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
Subject: Re: [PATCH tip/core/rcu 10/10] rcu: Account for rcu_all_qs() in
 cond_resched()

On Sat, Dec 02, 2017 at 09:56:26AM +0100, Peter Zijlstra wrote:
> On Fri, Dec 01, 2017 at 11:21:44AM -0800, Paul E. McKenney wrote:
> > If cond_resched() returns false, then it has already invoked
> > rcu_all_qs().  This is also invoked (now redundantly) by
> > rcu_note_voluntary_context_switch().  This commit therefore changes
> > cond_resched_rcu_qs() to invoke rcu_note_voluntary_context_switch_lite()
> > instead of rcu_note_voluntary_context_switch() to avoid the redundant
> > invocation of rcu_all_qs().
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  include/linux/rcupdate.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index a6ddc42f87a5..7bd8b5a6db10 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -197,7 +197,7 @@ static inline void exit_tasks_rcu_finish(void) { }
> >  #define cond_resched_rcu_qs() \
> >  do { \
> >  	if (!cond_resched()) \
> > -		rcu_note_voluntary_context_switch(current); \
> > +		rcu_note_voluntary_context_switch_lite(current); \
> >  } while (0)
> >  
> 
> Maybe I'm confused, but why are we keeping cond_resched_rcu_qs() around
> at all?

Because there are a few key places within RCU and rcutorture that need it.
Without it, there are scenarios where the new cond_resched() never gets
activated, and thus doesn't take effect.

The key point is that with this series in place, it should not be necessary
to use cond_resched_rcu_qs() outside of kernel/rcu and kernel/torture.c.
Which is a valuable step forward, right?

							Thanx, Paul