Re: [RFC PATCH v2 2/3] pstore: Add register read/write{b,w,l,q} tracing support

From: Sai Prakash Ranjan
Date: Tue Aug 28 2018 - 09:17:47 EST

On 8/27/2018 9:45 PM, Steven Rostedt wrote:
On Sat, 25 Aug 2018 12:54:07 +0530
Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx> wrote:

Ftrace does not trace __raw{read,write}{b,l,w,q}() functions. I am not
sure why and how it is filtered out because I do not see any notrace
flag in those functions, maybe that whole directory is filtered out.
So adding this functionality to ftrace would mean removing the notrace
for these functions i.e., something like using
__raw{read,write}{b,l,w,q}() as the available filter functions. Also
pstore ftrace does not filter functions to trace I suppose?

It's not traced because it is inlined. Simply make the __raw_read
function a normal function and it will be traced. And then you could
use ftrace to read the function.

If this has to be per arch, you can register your callback with the
REGS flags, and pt_regs will be passed to your callback function you
attached to __raw_read*() as if you inserted a break point at that
location, and you can get any reg data you want there.

Thank you very much for the information Steven. Ok so we can get function parameters with pt_regs.

Coming to the reason as to why it would be good to keep this separate
from ftrace would be:
* Ftrace can get ip and parent ip, but suppose we need extra data (field
data) as in above sample output we would not be able to get through ftrace.

As mentioned above, you can get regs (and ftrace is being expanded now
to get parameters of functions).

You mean there is another way to get parameters other than regs?

* Although this patch is for tracing register read/write, we can easily
add more functionality since we have dynamic_rtb api which can be hooked
to functions and start tracing events(IRQ, Context ID) something similar
to tracepoints.
Initially thought of having tracepoints for logging register read/write
but I do not know if we can export tracepoint data to pstore since the
main usecase is to debug unknown resets and hangs.

I don't know why not? We have read/write tracepoints for
read/write_msr() calls in x86.

Anything can add a hook to the callback of the tracepoints, and use
that infrastructure instead of creating yet another dynamic code
modification infrastructure.

Thanks for pointing out to read/write_msr, I checked it and was able to implement something similar for arm64. But still can we export tracepoint data to pstore because we need to debug reset cases and for that pstore is of real importance?. If so then it would be great to have various events logged into pstore which can be a lot of help for debugging.

Also with the current dynamic filtering of read/write(PATCH 3/3), it is a lot easier to filter register read/write since we use dynamic debug framework which provides file, function and line level filtering capacity. Maybe if we can add something like this to trace events it would be better?

- Sai Prakash