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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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