Re: [PATCH 1/2] rcu: Don't chase unnecessary quiescent states after extended grace periods

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Nov 24, 2010 at 09:45:08PM +0100, Frederic Weisbecker wrote:
> 2010/11/24 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> > I take it back.  I queued the following -- your code, but updated
> > comment and commit message.  Please let me know if I missed anything.
> >
> >                                                        Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 1d9d947bb882371a0877ba05207a0b996dcb38ee
> > Author: Frederic Weisbecker <fweisbec@gmail.com>
> > Date:   Wed Nov 24 01:31:12 2010 +0100
> >
> >    rcu: Don't chase unnecessary quiescent states after extended grace periods
> >
> >    When a CPU is in an extended quiescent state, including offline and
> >    dyntick-idle mode, other CPUs will detect the extended quiescent state
> >    and respond to the the current grace period on that CPU's behalf.
> >    However, the locking design prevents those other CPUs from updating
> >    the first CPU's rcu_data state.
> >
> >    Therefore, when this CPU exits its extended quiescent state, it must
> >    update its rcu_data state.  Because such a CPU will usually check for
> >    the completion of a prior grace period before checking for the start of a
> >    new grace period, the rcu_data ->completed field will be updated before
> >    the rcu_data ->gpnum field.  This means that if RCU is currently idle,
> >    the CPU will usually enter __note_new_gpnum() with ->completed set to
> >    the current grace-period number, but with ->gpnum set to some long-ago
> >    grace period number.  Unfortunately, __note_new_gpnum() will then insist
> >    that the current CPU needlessly check for a new quiescent state.  This
> >    checking can result in this CPU needlessly taking several scheduling-clock
> >    interrupts.
> 
> 
> So I'm all ok for the commit and the comments updated. But just a doubt about
> the about sentence.
> 
> The effect seems more that there will be one extra softirq. But not an
> extra tick
> because before sleeping, the CPU will check rcu_needs_cpu() which
> doesn't check for
> the need of noting a quiescent state, IIRC...
> 
> And I think the softirq will be only raised on the next tick.
> 
> Hm?

Good point!  This paragraph now reads:

	Therefore, when this CPU exits its extended quiescent state,
	it must update its rcu_data state.  Because such a CPU will
	usually check for the completion of a prior grace period
	before checking for the start of a new grace period, the
	rcu_data ->completed field will be updated before the rcu_data
	->gpnum field.	This means that if RCU is currently idle, the
	CPU will usually enter __note_new_gpnum() with ->completed set
	to the current grace-period number, but with ->gpnum set to some
	long-ago grace period number.  Unfortunately, __note_new_gpnum()
	will then insist that the current CPU needlessly check for a new
	quiescent state.  This checking can result in this CPU needlessly
	taking an additional softirq for unnecessary RCU processing.

Fair enough?

							Thanx, Paul

> >    This bug is harmless in most cases, but is a problem for users concerned
> >    with OS jitter for HPC applications or concerned with battery lifetime
> >    for portable SMP embedded devices.  This commit therefore makes the
> >    test in __note_new_gpnum() check for this situation and avoid the needless
> >    quiescent-state checks.
> >
> >    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> >    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >    Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> >    Cc: Ingo Molnar <mingo@elte.hu>
> >    Cc: Thomas Gleixner <tglx@linutronix.de>
> >    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> >    Cc: Steven Rostedt <rostedt@goodmis.org>
> >    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 5df948f..76cd5d2 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -616,8 +616,20 @@ static void __init check_cpu_stall_init(void)
> >  static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
> >  {
> >        if (rdp->gpnum != rnp->gpnum) {
> > -               rdp->qs_pending = 1;
> > -               rdp->passed_quiesc = 0;
> > +               /*
> > +                * Because RCU checks for the prior grace period ending
> > +                * before checking for a new grace period starting, it
> > +                * is possible for rdp->gpnum to be set to the old grace
> > +                * period and rdp->completed to be set to the new grace
> > +                * period.  So don't bother checking for a quiescent state
> > +                * for the rnp->gpnum grace period unless it really is
> > +                * waiting for this CPU.
> > +                */
> > +               if (rdp->completed != rnp->gpnum) {
> > +                       rdp->qs_pending = 1;
> > +                       rdp->passed_quiesc = 0;
> > +               }
> > +
> >                rdp->gpnum = rnp->gpnum;
> >        }
> >  }
> >
> --
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