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    SubjectRe: [PATCH tip/core/rcu 14/15] time: RCU permitted to stop idle entry via softirq
    On Thu, Aug 30, 2012 at 11:56:27AM -0700, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    >
    > The can_stop_idle_tick() function complains if a softirq vector is
    > raised too late in the idle-entry process, presumably in order to
    > prevent dangling softirq invocations from being delayed across the
    > full idle period, which might be indefinitely long -- and if softirq
    > was asserted any later than the call to this function, such a delay
    > might well happen.
    >
    > However, RCU needs to be able to use softirq to stop idle entry in
    > order to be able to drain RCU callbacks from the current CPU, which in
    > turn enables faster entry into dyntick-idle mode, which in turn reduces
    > power consumption. Because RCU takes this action at a well-defined
    > point in the idle-entry path, it is safe for RCU to take this approach.
    >
    > This commit therefore silences the error message that is sometimes
    > produced when the going-idle CPU suddenly finds that it has an RCU_SOFTIRQ
    > to process. The error message will continue to be issued for other
    > softirq vectors.
    >
    > Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
    > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

    Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    > ---
    > include/linux/interrupt.h | 2 ++
    > kernel/time/tick-sched.c | 3 ++-
    > 2 files changed, 4 insertions(+), 1 deletions(-)
    >
    > diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
    > index c5f856a..5e4e617 100644
    > --- a/include/linux/interrupt.h
    > +++ b/include/linux/interrupt.h
    > @@ -430,6 +430,8 @@ enum
    > NR_SOFTIRQS
    > };
    >
    > +#define SOFTIRQ_STOP_IDLE_MASK (~(1 << RCU_SOFTIRQ))
    > +
    > /* map softirq index to softirq name. update 'softirq_to_name' in
    > * kernel/softirq.c when adding a new softirq.
    > */
    > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    > index 024540f..4b1785a 100644
    > --- a/kernel/time/tick-sched.c
    > +++ b/kernel/time/tick-sched.c
    > @@ -436,7 +436,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
    > if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
    > static int ratelimit;
    >
    > - if (ratelimit < 10) {
    > + if (ratelimit < 10 &&
    > + (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
    > printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
    > (unsigned int) local_softirq_pending());
    > ratelimit++;
    > --
    > 1.7.8
    >


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