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

[Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>]

Hi Steve !

On 18/11/2024 21:20, Steven Rostedt wrote:
> 
> [ Added Tomas as he knows this code better than I do ]

Thanks !

> 
> On Mon, 18 Nov 2024 11:11:48 +0100
> Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org> wrote:
> 
>> Hi Steve,
>>
>> On 15/11/2024 20:55, Steven Rostedt wrote:
>>> On Fri, 15 Nov 2024 16:33:06 +0100
>>> Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org> wrote:
>>>    
>>>> Hi Steve,
>>>>
>>>> On 15/11/2024 16:25, Steven Rostedt wrote:
>>>>> On Fri, 15 Nov 2024 09:26:07 +0100
>>>>> Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org> wrote:
>>>>>       
>>>>>> Nevertheless it sounds like a really high latency for wake_up().
>>>>>>
>>>>>> I have a custom driver which basically gets an IRQ, and calls wake_up on
>>>>>> a read() call. This wake_up() on a high cpu usage can be more than 1ms !
>>>>>> Even with a fifo/99 priority for my kernel thread !
>>>>>>
>>>>>> I don't know if it rings any bell ?
>>>>>> I can obviously do more tests if it can help getting down to the issue :-).
>>>>>
>>>>> Try running timerlat.
>>>>
>>>> Thanks !
>>>> Here is what I get:
>>>> # echo timerlat > current_tracer
>>>> # echo 1 > events/osnoise/enable
>>>> # echo 25 > osnoise/stop_tracing_total_us
>>>> # tail -10 trace
>>>>                bash-224     [000] d.h..   153.268917: #77645 context  irq timer_latency     45056 ns
>>>>                bash-224     [000] dnh..   153.268987: irq_noise: timer:206  start 153.268879083 duration 93957 ns
>>>>                bash-224     [000] d....   153.269056: thread_noise:  bash:224 start 153.268905324 duration 71045 ns
>>>>          timerlat/0-271     [000] .....   153.269103: #77645 context thread timer_latency    230656 ns
>>>>                bash-224     [000] d.h..   153.269735: irq_noise: timer:206 start 153.269613847 duration 103558 ns
>>>>                bash-224     [000] d.h..   153.269911: #77646 context irq timer_latency     40640 ns
>>>>                bash-224     [000] dnh..   153.269982: irq_noise: timer:206 start 153.269875367 duration 93190 ns
>>>>                bash-224     [000] d....   153.270053: thread_noise: bash:224 start 153.269900969 duration 72709 ns
>>>>          timerlat/0-271     [000] .....   153.270100: #77646 context thread timer_latency    227008 ns
>>>>          timerlat/0-271     [000] .....   153.270155: timerlat_main: stop tracing hit on cpu 0
>>>>
>>>> It looks awful, right ?
>>>
>>> awful is relative ;-) If that was on x86, I would say it was bad.
>>>
>>> Also check out rtla (in tools/trace/rtla).
>>
>> Thanks ! I knew it only by name, so I watched a presentation recorded
>> during OSS summit given by Daniel Bristot de Oliveira who wrote it and
>> it is really impressive !
>>
>> I had to modify the source code a bit, as it does not compile with my
>> uclibc toolchain:
>> diff --git a/tools/tracing/rtla/Makefile.rtla
>> b/tools/tracing/rtla/Makefile.rtla
>> index cc1d6b615475..b22016a88d09 100644
>> --- a/tools/tracing/rtla/Makefile.rtla
>> +++ b/tools/tracing/rtla/Makefile.rtla
>> @@ -15,7 +15,7 @@ $(call allow-override,LD_SO_CONF_PATH,/etc/ld.so.conf.d/)
>>    $(call allow-override,LDCONFIG,ldconfig)
>>    export CC AR STRIP PKG_CONFIG LD_SO_CONF_PATH LDCONFIG
>>
>> -FOPTS          := -flto=auto -ffat-lto-objects -fexceptions
>> -fstack-protector-strong   \
>> +FOPTS          := -flto=auto -ffat-lto-objects -fexceptions \
>>                   -fasynchronous-unwind-tables -fstack-clash-protection
>>    WOPTS          := -O -Wall -Werror=format-security
>> -Wp,-D_FORTIFY_SOURCE=2             \
>>                   -Wp,-D_GLIBCXX_ASSERTIONS -Wno-maybe-uninitialized
> 
> I'm not sure what the consequence of the above would be. Perhaps Daniel
> just copied this from other code?
> 
>> diff --git a/tools/tracing/rtla/src/timerlat_u.c
>> b/tools/tracing/rtla/src/timerlat_u.c
>> index 01dbf9a6b5a5..92ad2388b123 100644
>> --- a/tools/tracing/rtla/src/timerlat_u.c
>> +++ b/tools/tracing/rtla/src/timerlat_u.c
>> @@ -15,10 +15,16 @@
>>    #include <pthread.h>
>>    #include <sys/wait.h>
>>    #include <sys/prctl.h>
>> +#include <sys/syscall.h>
>>
>>    #include "utils.h"
>>    #include "timerlat_u.h"
>>
>> +static inline pid_t gettid(void)
>> +{
>> +       return syscall(SYS_gettid);
>> +}
>> +
>>    /*
>>     * This is the user-space main for the tool timerlatu/ threads.
>>     *
>> diff --git a/tools/tracing/rtla/src/utils.c b/tools/tracing/rtla/src/utils.c
>> index 9ac71a66840c..b754dc1016a4 100644
>> --- a/tools/tracing/rtla/src/utils.c
>> +++ b/tools/tracing/rtla/src/utils.c
>> @@ -229,6 +229,9 @@ long parse_ns_duration(char *val)
>>    #elif __s390x__
>>    # define __NR_sched_setattr    345
>>    # define __NR_sched_getattr    346
>> +#elif __m68k__
>> +# define __NR_sched_setattr    349
>> +# define __NR_sched_getattr    350
>>    #endif
>>
>>    #define SCHED_DEADLINE         6
>>
>> But it is not enough, as executing rtla fails with a segfault.
>> I can dump a core, but I could not manage to build gdb for my board so I
>> can't debug it (I don't know how to debug a coredump without gdb !).
> 
> printf()!  That's how I debug things without gdb ;-)

Indeed printf gave me clues !
It appears to be a bug in libtracefs (v1.8.1). rtla segfaults when 
calling tracefs_local_events() in trace_instance_init().

Debugging libtracefs pointed me to the load_events() function, and the 
segfault happens after tep_parse_event() is called for 
"/sys/kernel/debug/tracing/events/vmscan/mm_vmscan_write_folio/format".

Going through the calls I get to event_read_print_args().
I changed libtraceevent log level to get the warnings, and it says:
libtraceevent: Resource temporarily unavailable
   unknown op '.'
Segmentation fault

JM