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    SubjectRe: [PATCH 2/2] rcu: Stop checking quiescent states after grace period completion from remote
    On Wed, Nov 24, 2010 at 01:31:13AM +0100, Frederic Weisbecker wrote:
    > After a CPU starts to chase its quiescent states by setting
    > rdp->qs_pending to 1, it can still enter into an extended
    > quiescent state and then another CPU will take care of this
    > and complete the grace period if necessary.
    >
    > rcu_report_qs_rdp() currently doesn't handle well this case
    > and considers it must try later to notify its quiescent state.
    >
    > However if the last grace period has been completed there is
    > nothing left to do for the current CPU.
    >
    > It means that until a next grace period starts, the CPU that
    > runs into that case will keep chasing its own quiescent states
    > by raising a softirq on every tick for no good reason.
    >
    > This can take a while before a new grace period starts and
    > this time slice is covered by spurious softirqs and other
    > kinds of rcu checks.
    >
    > Fix this by resetting rdp->qs_pending if the last grace
    > period has been completed by a remote CPU while we were
    > in an extended quiescent state.

    This one looks very good, at least at first glance!!! Queued.

    Thanx, Paul

    > 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>
    > ---
    > kernel/rcutree.c | 9 +++++++++
    > 1 files changed, 9 insertions(+), 0 deletions(-)
    >
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index 5f038a1..f287eaa 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -937,6 +937,15 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long las
    > * race occurred.
    > */
    > rdp->passed_quiesc = 0; /* try again later! */
    > +
    > + /*
    > + * Another CPU may have taken care of us if we were in an
    > + * extended quiescent state, in which case we don't need
    > + * to continue to track anything.
    > + */
    > + if (rnp->gpnum == rnp->completed)
    > + rdp->qs_pending = 0;
    > +
    > raw_spin_unlock_irqrestore(&rnp->lock, flags);
    > return;
    > }
    > --
    > 1.7.1
    >
    > --
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