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Date: Wed, 24 Nov 2010 13:19:37 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Lai Jiangshan <laijs@...fujitsu.com>,
Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/2] rcu: Don't chase unnecessary quiescent states
after extended grace periods
On Wed, Nov 24, 2010 at 09:45:08PM +0100, Frederic Weisbecker wrote:
> 2010/11/24 Paul E. McKenney <paulmck@...ux.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@...il.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@...il.com>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Lai Jiangshan <laijs@...fujitsu.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.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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