On 24 January 2013 17:44, Frederic Weisbecker fweisbec@gmail.com wrote:
2012/12/3 Vincent Guittot vincent.guittot@linaro.org:
With the coupled cpuidle driver (but probably also with other drivers), a CPU loops in a temporary safe state while waiting for other CPUs of its cluster to be ready to enter the coupled C-state. If an IRQ or a softirq occurs, the CPU will stay in this internal loop if there is no need to resched. The SCHED softirq clears the NOHZ and increases nr_busy_cpus. If there is no need to resched, we will not call set_cpu_sd_state_idle because of this internal loop in a cpuidle state. We have to call set_cpu_sd_state_idle in tick_nohz_irq_exit which is used to handle such situation.
I'm a bit confused with this.
set_cpu_sd_state_busy() is only called from nohz_kick_needed(). And it checks idle_cpu() before doing anything. So if no task is going to be scheduled, idle_cpu() prevents from calling set_cpu_sd_state_busy().
I'm probably missing something.
Hi Frederic
I can't find back the trace that i had saved with the issue but IIRC the sequence is: The CPU is kicked for ILB The wake_list of the CPU becomes not empty so cpu id not idle CPU wakes up, updates is timer framework and call nohz_kick_needed the execute the ILB sequence we don't go out of the cpuidle driver function because we don't need to resched so we don't clear the busy state
I'm going to look for the saved trace to check the sequence above
Vincent
Thanks.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
kernel/time/tick-sched.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 955d35b..b8d74ea 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -570,6 +570,8 @@ void tick_nohz_irq_exit(void) if (!ts->inidle) return;
set_cpu_sd_state_idle();/* Cancel the timer because CPU already waken up from the C-states*/ menu_hrtimer_cancel(); __tick_nohz_idle_enter(ts);-- 1.7.9.5
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