Messages in this thread | | | Date | Wed, 24 Nov 2010 21:45:08 +0100 | Subject | Re: [PATCH 1/2] rcu: Don't chase unnecessary quiescent states after extended grace periods | From | Frederic Weisbecker <> |
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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?
> > 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; > } > } > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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