linux-kernel - Re: Ftrace vs perf user page fault statistics differences

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Date:   Wed, 14 Jun 2017 16:01:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Will Hawkins <whh8b@...ginia.edu>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Namhyung Kim <namhyung@...il.com>
Subject: Re: Ftrace vs perf user page fault statistics differences

On Wed, 14 Jun 2017 13:47:17 -0400
Will Hawkins <whh8b@...ginia.edu> wrote:


> > That's how trace-cmd parses it.  
> 
> In the kernel version that I am running (again, pretty old) I do not
> have this file. I do, however, have

It may be due to the kernel version. It gets the functions
from /proc/kallsyms. That could have have an issue. Although, I have
this too:

# grep per_cpu_start /proc/kallsyms 
0000000000000000 A __per_cpu_start

But I don't see it being converted in my report.

Hmm, it's not saved. Interesting:

 trace-cmd report -f

to see the list of saved functions. I need to figure out why it does
for you, but not for me.

> 
> /sys/kernel/debug/tracing/events/exceptions/page_fault_user/format
> 
> and the contents are:
> 
> name: page_fault_user
> ID: 79
> format:
>     field:unsigned short common_type;    offset:0;    size:2;    signed:0;
>     field:unsigned char common_flags;    offset:2;    size:1;    signed:0;
>     field:unsigned char common_preempt_count;    offset:3;    size:1;signed:0;
>     field:int common_pid;    offset:4;    size:4;    signed:1;
> 
>     field:unsigned long address;    offset:8;    size:8;    signed:0;
>     field:unsigned long ip;    offset:16;    size:8;    signed:0;
>     field:unsigned long error_code;    offset:24;    size:8;    signed:0;
> 
> print fmt: "address=%pf ip=%pf error_code=0x%lx", (void
> *)REC->address, (void *)REC->ip, REC->error_code
> 
> Again, this looks like exactly what I would expect since address has
> the cr2 value in that function. Plus, we know that the raw value is
> correct. I suppose that the "symbolification" of that value is done in
> trace-cmd, right? So, perhaps that is where I should start looking for
> the problem?
> 
> I definitely want to follow up on this and help where I can. That
> said, I think I am satisfied with "our" (really, your) answer to the
> original problem.

It has something to do with the reading of the function names
in /proc/kallsyms.

Ah, I bet it's a change in the kernel. A recent update to trace-cmd was
to ignore functions in kallsyms of type "A" (which you can see is what
I have above).

What do you have when you do:

 sudo grep per_cpu_start /proc/kallsyms

May or may not need sudo depending on if the kernel lets non root have
access to kallsyms.


-- Steve