Re: [Adeos-main] [PATCH] I-pipe tracer for ARM

[Jan Kiszka <jan.kiszka@domain.hid>]

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Sebastian Smolorz wrote:
> Jan Kiszka wrote:
>> Sebastian Smolorz wrote:
>>> Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> Gilles Chanteperdrix wrote:
>>>>>> Jan Kiszka wrote:
>>>>>>> Sebastian Smolorz wrote:
>>>>>>>> Jan Kiszka wrote:
>>>>>>>>> Are you tracing into vmalloc'ed memory?
>>>>>>>> Yes, without CONFIG_IPIPE_TRACE_VMALLOC the system does not boot.
>>>>>>> Hmm, makes me wonder of some laziness of the page mapping or a
>>>>>>> missing lock-against-swapping causes this. Could you have a closer
>>>>>>> look at the tracer code if we are lacking some magic for the vmalloc
>>>>>>> trace buffer (compare to the xnheap code, e.g.)?
>>>>>> This is a known issue, the ARM architecture lacks the set_pgdir
>>>>>> function, needed in mm/vmalloc.c to workaround the lazy page mapping
>>>>>> of vmalloced areas.
>>>>> Would vmalloc+memset help to be safe for the remaining system runtime?
>>>> Probably not, the vmalloced area is added only to the page table of the
>>>> process that calls vmalloc, it is added to other processes table page
>>>> only once they use it.
>>> Are there any alternatives? Or do we have to live with this restriction?
>> Try if smaller CONFIG_IPIPE_TRACE_SHIFT makes it boot without
>> CONFIG_IPIPE_TRACE_VMALLOC.
> 
> I already tried that but it didn't help. First with IPIPE_TRACE_SHIFT set to 
> 10 and after modifying Kconfig.debug with a value of 5. Booting impossible.

What about starting disabled and arming the tracer later via
/proc/ipipe/trace/enable? This sounds like it's not a size issue
(TRACE_SHIFT=5 is ridiculous small), rather some memory section is
accessed too early.

If it works we could postpone enabling to __ipipe_init_tracer by default
(like it is done for TRACE_VMALLOC). I cannot imagine use cases for the
tracer ATM that would benefit from early tracing.

Jan


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