Re: Fix 80d20d35af1e ("nohz: Fix local_timer_softirq_pending()") may have revealed another problem

[Heiner Kallweit]

On 24.01.2019 20:37, Heiner Kallweit wrote:
> On 16.01.2019 07:24, Frederic Weisbecker wrote:
>> On Fri, Dec 28, 2018 at 12:11:12AM +0100, Heiner Kallweit wrote:
>>>
>>> # tracer: nop
>>> #
>>> #                              _-----=> irqs-off
>>> #                             / _----=> need-resched
>>> #                            | / _---=> hardirq/softirq
>>> #                            || / _--=> preempt-depth
>>> #                            ||| /     delay
>>> #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
>>> #              | |       |   ||||       |         |
>>>           <idle>-0     [001] d.h2  1479.099092: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.099098: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.099106: softirq_raise: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.099114: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.099120: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.099121: softirq_entry: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.099134: softirq_exit: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.099135: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.099141: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.100094: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.100109: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.100116: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.101091: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.101113: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.101118: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.102094: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.102114: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.102121: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.103091: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.103097: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.103105: softirq_raise: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.103114: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.103118: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.103119: softirq_entry: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.103131: softirq_exit: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.103132: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.103138: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.105092: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.105115: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.105119: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.106092: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.106112: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] .Ns2  1479.106144: softirq_exit: vec=9 [action=RCU]
>>>          cpuhp/1-13    [001] d..2  1479.106279: timer_cancel: timer=0000000009a25653
>>>           <idle>-0     [001] d.h2  1479.106965: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.106969: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.106974: softirq_raise: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.106981: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.106984: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.106985: softirq_entry: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.106994: softirq_exit: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.106995: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.106999: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.107996: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.108010: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.108014: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.109009: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.109013: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.109024: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.109028: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.109028: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.109033: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.110013: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.110033: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] ..s2  1479.110040: softirq_exit: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.111011: softirq_raise: vec=1 [action=TIMER]
>>>           <idle>-0     [001] d.h2  1479.111017: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] d.h2  1479.111026: softirq_raise: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.111035: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.111040: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] ..s2  1479.111040: softirq_entry: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.111052: softirq_exit: vec=7 [action=SCHED]
>>>           <idle>-0     [001] ..s2  1479.111052: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] .Ns2  1479.111079: softirq_exit: vec=9 [action=RCU]
>>>          cpuhp/1-13    [001] dNh2  1479.112930: softirq_raise: vec=1 [action=TIMER]
>>>          cpuhp/1-13    [001] dNh2  1479.112935: softirq_raise: vec=9 [action=RCU]
>>>           <idle>-0     [001] d..1  1479.113077: <stack trace>
>>>  => can_stop_idle_tick.isra.14
>>>  => tick_nohz_get_sleep_length
>>>  => menu_select
>>>  => cpuidle_select
>>>  => do_idle
>>>  => cpu_startup_entry
>>>  => start_secondary
>>>  => secondary_startup_64
>>>           <idle>-0     [001] .Ns2  1479.113110: softirq_entry: vec=1 [action=TIMER]
>>>           <idle>-0     [001] .Ns2  1479.113114: softirq_exit: vec=1 [action=TIMER]
>>>           <idle>-0     [001] .Ns2  1479.113115: softirq_entry: vec=9 [action=RCU]
>>>           <idle>-0     [001] .Ns2  1479.113139: softirq_exit: vec=9 [action=RCU]
>>
>>
>> So, I'm being very doubtful now actually.
>>
>> First of all, I realize my patch only fixes things halfway. Only softirq raised from
>> hardirqs are going to be handled after ksoftirqd is parked. If it's raised outside
>> interrupts, the softirq is going to be ignored. If other smpboot kthreads, following
>> ksoftirqd parking, raise softirqs from their park() callback we should at least warn
>> that we are ignoring that.
>>
>> Also, I'm still not sure if you're running threaded IRQs. Very likely not as
>> the softirqs in your traces are executed in idle, which means from an IRQ interrupting
>> idle. You haven't passed "threadirqs" in your kernel parameters, right?
>> In any case we need to figure out why your softirqs raised at 1479.112930 and
>> 1479.112935 don't get handled.
>>
>> Finally I don't understand why you have so many timer softirqs and yet no timer traces
>> at all, I turned them on in the script I gave you. So something weird is happening with
>> the timers code. At best we are wasting lots of CPU cycles with those empty softirqs, at
>> worst we are miscalculating some expiration somewhere, possibly ignoring a timer.
>>
>> So I'll need to cook up an updated debug patch for you to trigger if you don't
>> mind.
>>
> Hi Frederic,
> is the update debug patch still to come?
> 
>> Thanks.
>>
> Heiner
> 
Hi Frederic,

before it's forgotten: If I can test more, just let me know.

Heiner