On Thu, Aug 30, 2012 at 11:56:27AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mcken...@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.di...@gmail.com>
> Signed-off-by: Paul E. McKenney <paul.mcken...@linaro.org>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Tested-by: Sedat Dilek <sedat.di...@gmail.com>
Reviewed-by: Josh Triplett <j...@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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