Re: [PATCH] stm class: Document the stm_ftrace

[Chunyan Zhang]

On 21 March 2017 at 15:33, Alexander Shishkin
<alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
> On 21 March 2017 at 07:57, Chunyan Zhang <zhang.lyra@xxxxxxxxx> wrote:
>> On 20 March 2017 at 19:09, Alexander Shishkin
>> <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
>>> Chunyan Zhang <zhang.lyra@xxxxxxxxx> writes:
>>>
>>>> Hi Alex,
>>>>
>>>> On 20 March 2017 at 16:49, Alexander Shishkin
>>>> <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
>>>>> Hi Chunyan,
>>>>>
>>>>> A couple of clarifications: iirc this applies to the function tracer
>>>>> of ftrace, right? Does it make sense to mention that? Also, are you
>>>>
>>>> Right, only applies to the function tracer currently (actually only
>>>> function address and parent function address of Function tracer is
>>>> recorded into STM, I mean it doesn't include like "pid" "task name"
>>>> "cpu-id" these information right now). It makes sense to mention
>>>> function tracer, I will address that.
>>>
>>> Thanks!
>>>
>>>>> planning to support other ftrace payloads like trace_printk()s?
>>>>
>>>> No plan so far, but I think I can consider to do that, it depends on
>>>> how many people think that are helpful.
>>>> What do you think?
>>>
>>> Well, I myself almost never use function tracer, but I do use
>>> tracepoints/trace_printk()s. I'm *guessing* that everybody who's
>>
>> In fact I had implemented exporting tracepoints to STM and tried
>> upstreaming that, but Steven Rostedt and Ingo expressed their worries
>> on that would introduce a considerable impact on Ftrace fast path
>> since a tracepoint basically was a string which was too long to be
>> written to STM with some acceptable impact on fast path, so I stopped
>> upstreaming that feature.
>
> Did we ever consider writing a string pointer (or even on offset into

Regarding to the pointer, you mean event pointer in Ftrace ring
buffer? I considered recording trace_events' ring buffer address which
their meta data is stored, my concern was how we should do if ring
buffer overwriting happened.

> the corresponding section, to avoid KASLR fun) instead of the actual
> string? This would require a kernel binary on the decoding end,
> though.

Decoding function traces supported currently from STM also needs
kernel binary :)

Thanks,
Chunyan

>
> Regards,
> --
> Alex