Linux kernel ftrace irqsoff tracer generates strange output - c

Question

I was analyzing the interrupt latency in my system( Freescale ARM MX6Q with linux kernel version 3.0.35), I am using the irqsoff tracer, and this is the output I'm getting for the biggest irq off latency:
# tracer: irqsoff
#
# irqsoff latency trace v1.1.5
# --------------------------------------------------------------------
# latency: 2770 us, #89/89, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
# -----------------
# | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
# -----------------
# => started at: tick_oneshot_mode_active
# => ended at: default_idle
#
#
# _------=> CPU#
# / _-----=> irqs-off
# | / _----=> need-resched
# || / _---=> hardirq/softirq
# ||| / _--=> preempt-depth
# |||| / delay
# cmd pid ||||| time | caller
# \ / ||||| \ | /
<idle>-0 0d..1 1us : tick_oneshot_mode_active
<idle>-0 0d..1 2us : clockevents_notify <-arch_idle
<idle>-0 0d..1 3us : _raw_spin_lock_irqsave <-clockevents_notify
<idle>-0 0d..1 5us : __raw_spin_lock_irqsave <-_raw_spin_lock_irqsave
<idle>-0 0d..1 6us : add_preempt_count <-__raw_spin_lock_irqsave
<idle>-0 0d..2 7us : clockevents_do_notify <-clockevents_notify
<idle>-0 0d..2 8us : raw_notifier_call_chain <-clockevents_do_notify
<idle>-0 0d..2 10us : __raw_notifier_call_chain <-raw_notifier_call_chain
<idle>-0 0d..2 11us : notifier_call_chain <-__raw_notifier_call_chain
<idle>-0 0d..2 12us : tick_notify <-notifier_call_chain
<idle>-0 0d..2 13us : tick_broadcast_oneshot_control <-tick_notify
<idle>-0 0d..2 15us : _raw_spin_lock_irqsave <-tick_broadcast_oneshot_control
<idle>-0 0d..2 16us : __raw_spin_lock_irqsave <-_raw_spin_lock_irqsave
<idle>-0 0d..2 17us+: add_preempt_count <-__raw_spin_lock_irqsave
<idle>-0 0d..3 19us : clockevents_set_mode <-tick_broadcast_oneshot_control
<idle>-0 0d..3 20us : twd_set_mode <-clockevents_set_mode
<idle>-0 0d..3 21us : irq_modify_status <-gic_disable_ppi
<idle>-0 0d..3 22us : __irq_get_desc_lock <-irq_modify_status
<idle>-0 0d..3 24us : irq_to_desc <-__irq_get_desc_lock
<idle>-0 0d..3 25us : _raw_spin_lock_irqsave <-__irq_get_desc_lock
<idle>-0 0d..3 26us : __raw_spin_lock_irqsave <-_raw_spin_lock_irqsave
<idle>-0 0d..3 27us : add_preempt_count <-__raw_spin_lock_irqsave
<idle>-0 0d..4 29us : __irq_put_desc_unlock <-irq_modify_status
<idle>-0 0d..4 30us : _raw_spin_unlock_irqrestore <-__irq_put_desc_unlock
<idle>-0 0d..4 31us : sub_preempt_count <-_raw_spin_unlock_irqrestore
<idle>-0 0d..3 32us : irq_get_irq_data <-gic_disable_ppi
<idle>-0 0d..3 34us : irq_to_desc <-irq_get_irq_data
<idle>-0 0d..3 35us : gic_mask_irq <-gic_disable_ppi
<idle>-0 0d..3 36us : _raw_spin_lock <-gic_mask_irq
<idle>-0 0d..3 37us : __raw_spin_lock <-_raw_spin_lock
<idle>-0 0d..3 39us : add_preempt_count <-__raw_spin_lock
<idle>-0 0d..4 40us : _raw_spin_unlock <-gic_mask_irq
<idle>-0 0d..4 41us+: sub_preempt_count <-_raw_spin_unlock
<idle>-0 0d..3 43us : _raw_spin_unlock_irqrestore <-tick_broadcast_oneshot_control
<idle>-0 0d..3 44us : sub_preempt_count <-_raw_spin_unlock_irqrestore
<idle>-0 0d..2 45us+: _raw_spin_unlock_irqrestore <-clockevents_notify
<idle>-0 0d..2 47us : sub_preempt_count <-_raw_spin_unlock_irqrestore
<idle>-0 0d..1 48us+: mxc_cpu_lp_set <-arch_idle
<idle>-0 0d..1 50us+: gpc_mask_single_irq <-mxc_cpu_lp_set
<idle>-0 0d..1 51us+: gpc_mask_single_irq <-mxc_cpu_lp_set
<idle>-0 0d..1 53us+: arch_idle_multi_core <-arch_idle
<idle>-0 0d..1 55us!: ca9_do_idle <-arch_idle_multi_core
<idle>-0 0d..1 2709us : clockevents_notify <-arch_idle
<idle>-0 0d..1 2710us : _raw_spin_lock_irqsave <-clockevents_notify
<idle>-0 0d..1 2711us : __raw_spin_lock_irqsave <-_raw_spin_lock_irqsave
<idle>-0 0d..1 2712us+: add_preempt_count <-__raw_spin_lock_irqsave
<idle>-0 0d..2 2714us : clockevents_do_notify <-clockevents_notify
<idle>-0 0d..2 2715us : raw_notifier_call_chain <-clockevents_do_notify
<idle>-0 0d..2 2716us : __raw_notifier_call_chain <-raw_notifier_call_chain
<idle>-0 0d..2 2717us : notifier_call_chain <-__raw_notifier_call_chain
<idle>-0 0d..2 2718us : tick_notify <-notifier_call_chain
<idle>-0 0d..2 2720us : tick_broadcast_oneshot_control <-tick_notify
<idle>-0 0d..2 2721us : _raw_spin_lock_irqsave <-tick_broadcast_oneshot_control
<idle>-0 0d..2 2722us : __raw_spin_lock_irqsave <-_raw_spin_lock_irqsave
<idle>-0 0d..2 2723us : add_preempt_count <-__raw_spin_lock_irqsave
<idle>-0 0d..3 2725us : clockevents_set_mode <-tick_broadcast_oneshot_control
<idle>-0 0d..3 2726us : twd_set_mode <-clockevents_set_mode
<idle>-0 0d..3 2727us : irq_modify_status <-gic_enable_ppi
<idle>-0 0d..3 2729us : __irq_get_desc_lock <-irq_modify_status
<idle>-0 0d..3 2730us : irq_to_desc <-__irq_get_desc_lock
<idle>-0 0d..3 2731us : _raw_spin_lock_irqsave <-__irq_get_desc_lock
<idle>-0 0d..3 2732us : __raw_spin_lock_irqsave <-_raw_spin_lock_irqsave
<idle>-0 0d..3 2733us+: add_preempt_count <-__raw_spin_lock_irqsave
<idle>-0 0d..4 2738us+: __irq_put_desc_unlock <-irq_modify_status
<idle>-0 0d..4 2739us : _raw_spin_unlock_irqrestore <-__irq_put_desc_unlock
<idle>-0 0d..4 2741us : sub_preempt_count <-_raw_spin_unlock_irqrestore
<idle>-0 0d..3 2742us : irq_get_irq_data <-gic_enable_ppi
<idle>-0 0d..3 2743us : irq_to_desc <-irq_get_irq_data
<idle>-0 0d..3 2745us : gic_unmask_irq <-gic_enable_ppi
<idle>-0 0d..3 2746us : _raw_spin_lock <-gic_unmask_irq
<idle>-0 0d..3 2747us : __raw_spin_lock <-_raw_spin_lock
<idle>-0 0d..3 2748us+: add_preempt_count <-__raw_spin_lock
<idle>-0 0d..4 2750us : _raw_spin_unlock <-gic_unmask_irq
<idle>-0 0d..4 2751us+: sub_preempt_count <-_raw_spin_unlock
<idle>-0 0d..3 2753us : ktime_get <-tick_broadcast_oneshot_control
<idle>-0 0d..3 2754us+: clocksource_mmio_readl_up <-ktime_get
<idle>-0 0d..3 2756us : tick_program_event <-tick_broadcast_oneshot_control
<idle>-0 0d..3 2757us : tick_dev_program_event <-tick_program_event
<idle>-0 0d..3 2758us : ktime_get <-tick_dev_program_event
<idle>-0 0d..3 2759us+: clocksource_mmio_readl_up <-ktime_get
<idle>-0 0d..3 2761us : clockevents_program_event <-tick_dev_program_event
<idle>-0 0d..3 2762us : twd_set_next_event <-clockevents_program_event
<idle>-0 0d..3 2764us : _raw_spin_unlock_irqrestore <-tick_broadcast_oneshot_control
<idle>-0 0d..3 2765us+: sub_preempt_count <-_raw_spin_unlock_irqrestore
<idle>-0 0d..2 2767us : _raw_spin_unlock_irqrestore <-clockevents_notify
<idle>-0 0d..2 2768us : sub_preempt_count <-_raw_spin_unlock_irqrestore
<idle>-0 0d..1 2769us+: default_idle
<idle>-0 0d..1 2772us+: trace_hardirqs_on <-default_idle
<idle>-0 0d..1 2773us : <stack trace>
=> trace_hardirqs_on
=> default_idle
=> cpu_idle
=> rest_init
=> start_kernel
=>
So according to ftrace I have a pretty big latency, it looks as if the interrupts are disabled as long as the CPU is idle. So I thought surely this cannot be true, so I looked more closely at function tick_oneshot_mode_active:
int tick_oneshot_mode_active(void)
{
unsigned long flags;
int ret;
local_irq_save(flags);
ret = __this_cpu_read(tick_cpu_device.mode) == TICKDEV_MODE_ONESHOT;
local_irq_restore(flags);
return ret;
}
So in the macro local_irq_save, the interrupts are disabled and the function trace_hardirqs_off() is called which registers that the interrupts were disabled:
#define local_irq_save(flags) \
do { \
raw_local_irq_save(flags); \
trace_hardirqs_off(); \
} while (0)
After checking the device mode the interrupts are enabled, now here it is strange:
#define local_irq_restore(flags) \
do { \
if (raw_irqs_disabled_flags(flags)) { \
raw_local_irq_restore(flags); \
trace_hardirqs_off(); \
} else { \
trace_hardirqs_on(); \
raw_local_irq_restore(flags); \
} \
} while (0)
In my case the code goes over the first branch:
if (raw_irqs_disabled_flags(flags)) { \
raw_local_irq_restore(flags); \
trace_hardirqs_off(); \
It looks as if even though the interrupts are enabled back, function trace_hardirqs_on is not called but trace_hardirqs_off(). Why is this happening, do I really have this big latency?

Answer 1

I faced with the same problem, and I patch kernel with this:
https://lkml.org/lkml/2014/5/25/83
after that idle disappear from trace.