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    SubjectRe: [PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs it
    On Mon, Aug 15, 2011 at 05:52:11PM +0200, Frederic Weisbecker wrote:
    > If RCU is waiting for the current CPU to complete a grace
    > period, don't turn off the tick. Unlike dynctik-idle, we

    s/dynctik/dyntick/ ;-)

    > are not necessarily going to enter into rcu extended quiescent
    > state, so we may need to keep the tick to note current CPU's
    > quiescent states.

    One question below...

    Thanx, Paul

    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Anton Blanchard <anton@au1.ibm.com>
    > Cc: Avi Kivity <avi@redhat.com>
    > Cc: Ingo Molnar <mingo@elte.hu>
    > Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
    > Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
    > Cc: Paul Menage <menage@google.com>
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Stephen Hemminger <shemminger@vyatta.com>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com>
    > ---
    > include/linux/rcupdate.h | 1 +
    > kernel/rcutree.c | 3 +--
    > kernel/sched.c | 14 ++++++++++++++
    > 3 files changed, 16 insertions(+), 2 deletions(-)
    >
    > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    > index 99f9aa7..55a482a 100644
    > --- a/include/linux/rcupdate.h
    > +++ b/include/linux/rcupdate.h
    > @@ -133,6 +133,7 @@ static inline int rcu_preempt_depth(void)
    > extern void rcu_sched_qs(int cpu);
    > extern void rcu_bh_qs(int cpu);
    > extern void rcu_check_callbacks(int cpu, int user);
    > +extern int rcu_pending(int cpu);
    > struct notifier_block;
    >
    > #ifdef CONFIG_NO_HZ
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index ba06207..0009bfc 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -205,7 +205,6 @@ int rcu_cpu_stall_suppress __read_mostly;
    > module_param(rcu_cpu_stall_suppress, int, 0644);
    >
    > static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
    > -static int rcu_pending(int cpu);
    >
    > /*
    > * Return the number of RCU-sched batches processed thus far for debug & stats.
    > @@ -1729,7 +1728,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
    > * by the current CPU, returning 1 if so. This function is part of the
    > * RCU implementation; it is -not- an exported member of the RCU API.
    > */
    > -static int rcu_pending(int cpu)
    > +int rcu_pending(int cpu)
    > {
    > return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
    > __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
    > diff --git a/kernel/sched.c b/kernel/sched.c
    > index 0e1aa4e..353a66f 100644
    > --- a/kernel/sched.c
    > +++ b/kernel/sched.c
    > @@ -2439,6 +2439,7 @@ DEFINE_PER_CPU(int, task_nohz_mode);
    > bool cpuset_nohz_can_stop_tick(void)
    > {
    > struct rq *rq;
    > + int cpu;
    >
    > rq = this_rq();
    >
    > @@ -2446,6 +2447,19 @@ bool cpuset_nohz_can_stop_tick(void)
    > if (rq->nr_running > 1)
    > return false;
    >
    > + cpu = smp_processor_id();
    > +
    > + /*
    > + * FIXME: will probably be removed soon as it's
    > + * already checked from tick_nohz_stop_sched_tick()
    > + */
    > + if (rcu_needs_cpu(cpu))
    > + return false;
    > +
    > + /* Is there a grace period to complete ? */
    > + if (rcu_pending(cpu))

    This is from a quiescent state for both RCU and RCU-bh, right?
    Or can their be RCU or RCU-bh read-side critical sections held
    across here? (It would be mildly bad if so.)

    But force_quiescent_state() will catch cases where RCU needs
    quiescent states from CPUs, so is this check really needed?

    > + return false;
    > +
    > return true;
    > }
    >
    > --
    > 1.7.5.4
    >


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