linux-kernel - Re: [PATCH RFC 0/2] Add basic tracing support for m68k

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Message-ID: <1fbfc00d-27c0-4486-816d-50291bebc842@gmail.com>
Date: Wed, 20 Nov 2024 07:25:29 +1300
From: Michael Schmitz <schmitzmic@...il.com>
To: Jean-Michel Hautbois <jeanmichel.hautbois@...eli.org>,
 Steven Rostedt <rostedt@...dmis.org>
Cc: linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Geert Uytterhoeven
 <geert@...ux-m68k.org>, Greg Ungerer <gerg@...ux-m68k.org>,
 Tomas Glozar <tglozar@...hat.com>
Subject: Re: [PATCH RFC 0/2] Add basic tracing support for m68k

Hi Steve,

On 20/11/24 07:06, Jean-Michel Hautbois wrote:
>
>> You reference two variables that are not part of the event:
>>
>>   "mem_map" and "m68k_memory[0].addr"
>>
>> Do these variables ever change? Because the TP_printk() part of the
>> TRACE_EVENT() macro is called a long time after the event is 
>> recorded. It
>> could be seconds, minutes, days or even months (and unlikely possibly
>> years) later.
>
> I am really not the best placed to answer.
> AFAIK, it sounds like those are never changing.

m68k_memory[0].addr never changes (that segment is usually where the 
kernel is loaded to, and can't be hotplugged).

mem_map is equal to NODE_DATA(0)->node_mem_map on m68k 
(mm/mm_init.c:__init alloc_node_mem_map()) and won't change either.

Cheers,

     Michael

>
>>
>> The event takes place and runs the TP_fast_assign() to record the 
>> event in
>> the ring buffer. Then some time later, when you read the "trace" 
>> file, the
>> TP_printk() portion gets run. If you wait months before reading that, 
>> it is
>> executed months later.
>>
>> Now you have "mem_map" and "m68k_memory[0].addr" in that output that 
>> gets
>> run months after the fact. Are they constant throughout the boot?
>
> I don't know.
>
>> Now another issue is that user space has no idea what those values 
>> are. Now
>> user space can not print the values. Currently the code crashes 
>> because you
>> are the first one to reference a global value from a trace event 
>> print fmt.
>> That should probably be fixed to simply fail to parse the event and 
>> ignore
>> the print format logic (which defaults to just printing the raw fields).
>
> The patch you sent works...
> But, it fails a bit later:
> Dispatching timerlat u procs
> starting loop
> User-space timerlat pid 230 on cpu 0
> Segmentation fault
>
>
>>
>> -- Steve
>>
>
>