Philippe Gerum wrote:
> Jan Kiszka wrote:
>> Philippe Gerum wrote:
>>> Jan Kiszka wrote:
>>>> Philippe Gerum wrote:
>>>>> Jan Kiszka wrote:
>>>>>> Hi,
>>>>>>
>>>>>> the watchdog is currently broken in trunk ("zombie [...] would not
>>>>>> die..."). In fact, it should also be broken in older versions, but only
>>>>>> recent thread termination rework made this visible.
>>>>>>
>>>>>> When a Xenomai CPU hog is caught by the watchdog,
>>>>>> xnpod_delete_thread is
>>>>>> invoked, causing the current thread to be set in zombie state and
>>>>>> scheduled out. But as its Linux mate still exist, hell breaks loose
>>>>>> once
>>>>>> Linux tries to get rid of it (the Xenomai zombie is scheduled in
>>>>>> again).
>>>>>> In short: calling xnpod_delete_thread(<self>) for a shadow thread is
>>>>>> not
>>>>>> working, probably never worked cleanly.
>>>>> Nak, it is a regression introduced by the scheduler changes in 2.5.x.
>>>>> We should detect _any_ shadow thread that schedules out in primary
>>>>> mode then regains control in secondary mode like we do in the 2.4.x
>>>>> series, not only _relaxing_ shadow threads. It is perfectly valid to
>>>>> have the Linux task orphaned from the deletion of its shadow TCB
>>>>> until Xenomai notices the issue and reaps it; problem was that such
>>>>> regression prevented the nucleus to get the memo.
>>>>>
>>>>> The following patch should fix the issue:
>>>>>
>>>>>   Index: include/asm-generic/system.h
>>>>> ===================================================================
>>>>> --- include/asm-generic/system.h    (revision 4676)
>>>>> +++ include/asm-generic/system.h    (working copy)
>>>>> @@ -311,6 +311,11 @@
>>>>>       return !!s;
>>>>>   }
>>>>>
>>>>> +static inline int xnarch_root_domain_p(void)
>>>>> +{
>>>>> +    return rthal_current_domain == rthal_root_domain;
>>>>> +}
>>>>> +
>>>>>   #ifdef CONFIG_SMP
>>>>>
>>>>>   #define xnlock_get(lock)        __xnlock_get(lock  XNLOCK_DBG_CONTEXT)
>>>>> Index: ksrc/nucleus/pod.c
>>>>> ===================================================================
>>>>> --- ksrc/nucleus/pod.c    (revision 4676)
>>>>> +++ ksrc/nucleus/pod.c    (working copy)
>>>>> @@ -2137,7 +2137,7 @@
>>>>>   void __xnpod_schedule(struct xnsched *sched)
>>>>>   {
>>>>>       struct xnthread *prev, *next, *curr = sched->curr;
>>>>> -    int zombie, switched = 0, need_resched, relaxing;
>>>>> +    int zombie, switched = 0, need_resched, shadow;
>>>>>       spl_t s;
>>>>>
>>>>>       if (xnarch_escalate())
>>>>> @@ -2174,9 +2174,9 @@
>>>>>              next, xnthread_name(next));
>>>>>
>>>>>   #ifdef CONFIG_XENO_OPT_PERVASIVE
>>>>> -    relaxing = xnthread_test_state(prev, XNRELAX);
>>>>> +    shadow = xnthread_test_state(prev, XNSHADOW);
>>>>>   #else
>>>>> -    (void)relaxing;
>>>>> +    (void)shadow;
>>>>>   #endif /* CONFIG_XENO_OPT_PERVASIVE */
>>>>>
>>>>>       if (xnthread_test_state(next, XNROOT)) {
>>>>> @@ -2204,12 +2204,18 @@
>>>>>
>>>>>   #ifdef CONFIG_XENO_OPT_PERVASIVE
>>>>>       /*
>>>>> -     * Test whether we are relaxing a thread. In such a case, we
>>>>> -     * are here the epilogue of Linux' schedule, and should skip
>>>>> -     * xnpod_schedule epilogue.
>>>>> +     * Test whether we transitioned from primary mode to secondary
>>>>> +     * over a shadow thread. This may happen in two cases:
>>>>> +     *
>>>>> +     * 1) the shadow thread just relaxed.
>>>>> +     * 2) the shadow TCB has just been deleted, in which case
>>>>> +     * we have to reap the mated Linux side as well.
>>>>> +     *
>>>>> +     * In both cases, we are running over the epilogue of Linux's
>>>>> +     * schedule, and should skip our epilogue code.
>>>>>        */
>>>>> -    if (relaxing)
>>>>> -        goto relax_epilogue;
>>>>> +    if (shadow && xnarch_root_domain_p())
>>>>> +        goto shadow_epilogue;
>>>>>   #endif /* CONFIG_XENO_OPT_PERVASIVE */
>>>>>
>>>>>       switched = 1;
>>>>> @@ -2252,7 +2258,7 @@
>>>>>       return;
>>>>>
>>>>>   #ifdef CONFIG_XENO_OPT_PERVASIVE
>>>>> -      relax_epilogue:
>>>>> +      shadow_epilogue:
>>>>>       {
>>>>>           spl_t ignored;
>>>> Finally makes sense and works (but your posting was corrupted). Great.
>>>>
>>>>>> There are basically two approaches to fix it: The first one is to
>>>>>> find a
>>>>>> different way to kill (or only suspend?)
>>>>> Suspending the hog won't work, particularly when GDB is involved,
>>>>> because a pending non-lethal Linux signal may cause the suspended
>>>>> shadow to resume immediately for processing the signal, therefore
>>>>> defeating the purpose of the watchdog, leading to an infinite loop.
>>>>> This is why we moved from suspension to deletion upon watchdog
>>>>> trigger in 2.3 (2.2 used to suspend only).
>>>> Yes, that became clear to me in the meantime, too.
>>>>
>>>>>   the current shadow thread when
>>>>>> the watchdog strikes. The second one brought me to another issue: Raise
>>>>>> SIGKILL for the current thread and make sure that it can be
>>>>>> processed by
>>>>>> Linux (e.g. via xnpod_suspend_thread(<cpu-hog>). Unfortunately,
>>>>>> there is
>>>>>> no way to force a shadow thread into secondary mode to handle pending
>>>>>> Linux signals unless that thread issues a syscall once in a while. And
>>>>>> that raises the question if we shouldn't improve this as well while we
>>>>>> are on it.
>>>>>>
>>>>>> Granted, non-broken Xenomai user space threads always issue frequent
>>>>>> syscalls, otherwise the system would starve (and the watchdog would
>>>>>> come
>>>>>> around). On the other hand, delaying signals till syscall prologues is
>>>>>> different from plain Linux behaviour...
>>>>>>
>>>>>> Comments, ideas?
>>>>>>
>>>>> We probably need a two-stage approach: first record the thread was
>>>>> bumped out and suspend it from the watchdog handler to give Linux a
>>>>> chance to run again, then finish the work, killing it for good, next
>>>>> time the root thread is scheduled in on the same CPU.
>>>> That confuses me again: The watchdog issue is solved now, no? We are
>>>> only left with the scenario of breaking out of a user space loop of some
>>>> Xenomai thread via a Linux signal (which implies SMP - otherwise there
>>>> is no chance to raise the signal...).
>>>>
>>> If you first suspend the hog, then send it a lethal signal, you solve
>>> both issues: first Linux is allowed to run eventually, then your task
>>> won't be able to resume running the faulty code, but solely to process
>>> SIGKILL, which can be made pending early enough because the nucleus
>>> decides when Linux resumes.
>> I'm not interested in SIGKILL here, rather in SIGSTOP to do debugging.
>> That is currently impossible.
>>
>>>> Meanwhile I played with some light-weight approach to relax a thread
>>>> that received a signal (according to do_sigwake_event). Worked, but only
>>>> once due to a limitation (if not bug) of I-pipe x86: in __ipipe_run_isr,
>>>> it does not handle the case that a non-root handler may alter the
>>>> current domain, causing corruptions to the IPIPE_SYNC_FLAG states of the
>>>> involved domains.
>>> It is not a bug, this is wanted. ISR must neither change the current
>>> domain nor migrate CPU; allowing this would open Pandora's box.
>> OK, then please elaborate on this a bit more in the adeos-main thread
>> and explain why __ipipe_sync_stage currently reloads the domain.
>>
> 
> ipipe_cpudom_ptr() may be affected by CPU migration within the _root_ domain, 
> which does not mean that non-root domains are allowed to migrate and/or 
> change 
> domains.

ipd or ipipe_current_domain should not be affected by CPU migration, so
I still see no point in re-reading the current domain unless it actually
changes.

Jan

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