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

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Message-ID: <20241118152057.13042840@gandalf.local.home>
Date: Mon, 18 Nov 2024 15:20:57 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Jean-Michel Hautbois <jeanmichel.hautbois@...eli.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


[ Added Tomas as he knows this code better than I do ]

On Mon, 18 Nov 2024 11:11:48 +0100
Jean-Michel Hautbois <jeanmichel.hautbois@...eli.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@...eli.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@...eli.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 ;-)

-- Steve