Re: [PATCH v4 09/10] trace: platform/x86/intel/ifs: Add trace point to track Intel IFS operations

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 26 Apr 2022 16:53:13 -0700
"Luck, Tony" <tony.luck@intel.com> wrote:
> 
> I looked at the examples in samples/trace_events/trace-events-sample.h
> and tried to use this. But I'm doing something wrong because the
> compiler barfs on something defined but not used.
> 
> Maybe my problem is the TP_printk() in the DECLARE_EVENT_CLASS() that
> is over-ridden by DEFINE_EVENT_PRINT(). I wasn't at all sure what to
> put here ... or how to use the base tracepoint that doesn't have the
> printk() over-ridden.

Yeah, that could be confusing.

Basically, TRACE_EVENT() is simply defined as:

DECLARE_EVENT_CLASS()
DEFINE_EVENT();

So technically you could create the class and event with the same name,
and then you could create a second event on top of that. But I usually
suggest people explicitly specify the DECLARE_EVENT_CLASS() and
DEFINE_EVENT().

I would not do this until you have more than one event. The what you
would do is create one event where the print matches the
DECLARE_EVENT_CLASS() TP_printk(), and have that event defined with
just DEFINE_EVENT(). Then create other events with the
DEFINE_EVENT_PRINT().

> 
> I think I need my class to just save both the u64 values to the trace
> buffer. Then the different trace points will extract the bits they want
> and print in a user friendly way. While this increases space used in
> the trace buffer, these events are not crazy high frequency. Usually 
> one or two events per core with a gap 30 minutes or more between tests.
> 
> In my ".c" file the tracepoint looks like this using the name from
> DEFINE_EVENT_PRINT(), and now passing the full u64 values:
> 
> 	trace_ifs_status_saf(activate.data, status.data);
> 
> and my #include file looks like this:
> 
> ----------------------------------------------
> /* SPDX-License-Identifier: GPL-2.0 */
> #undef TRACE_SYSTEM
> #define TRACE_SYSTEM intel_ifs
> 
> #if !defined(_TRACE_IFS_H) || defined(TRACE_HEADER_MULTI_READ)
> #define _TRACE_IFS_H
> 
> #include <linux/ktime.h>
> #include <linux/tracepoint.h>
> 
> DECLARE_EVENT_CLASS(ifs_status,
> 
> 	TP_PROTO(u64 activate, u64 status),
> 
> 	TP_ARGS(activate, status),
> 
> 	TP_STRUCT__entry(
> 		__field(	u64,	activate	)
> 		__field(	u64,	status		)
> 	),
> 
> 	TP_fast_assign(
> 		__entry->activate = activate;
> 		__entry->status	= status;
> 	),
> 
> 	TP_printk("activate: %llx status: %llx",
> 		__entry->activate,
> 		__entry->status)
> );
> 
> DEFINE_EVENT_PRINT(ifs_status, ifs_status_saf,
> 	TP_PROTO(u64 activate, u64 status),
> 	TP_ARGS(activate, status),
> 	TP_printk("start: %.2x, stop: %.2x, status: %llx",
> 		((union ifs_scan *)&(__entry->activate))->start,
> 		((union ifs_scan *)&(__entry->activate))->stop,
> 		__entry->status)
> );
> 
> #endif /* _TRACE_IFS_H */
> 
> /* This part must be outside protection */
> #include <trace/define_trace.h>
> -----------------------------------------------------
> 
> GCC messages:
> 
> 
>   CC [M]  drivers/platform/x86/intel/ifs/runtest.o
> In file included from /home/agluck/GIT/mywork/include/trace/define_trace.h:102,
>                  from /home/agluck/GIT/mywork/include/trace/events/intel_ifs.h:44,
>                  from /home/agluck/GIT/mywork/drivers/platform/x86/intel/ifs/runtest.c:27:
> /home/agluck/GIT/mywork/include/trace/trace_events.h:426:13: warning: ‘print_fmt_ifs_status’ defined but not used [-Wunused-variable]
>   426 | static char print_fmt_##call[] = print;                                 \
>       |             ^~~~~~~~~~
> /home/agluck/GIT/mywork/include/trace/events/intel_ifs.h:11:1: note: in expansion of macro ‘DECLARE_EVENT_CLASS’
>    11 | DECLARE_EVENT_CLASS(ifs_status,
>       | ^~~~~~~~~~~~~~~~~~~
> In file included from /home/agluck/GIT/mywork/include/trace/define_trace.h:102,
>                  from /home/agluck/GIT/mywork/include/trace/events/intel_ifs.h:44,
>                  from /home/agluck/GIT/mywork/drivers/platform/x86/intel/ifs/runtest.c:27:
> /home/agluck/GIT/mywork/include/trace/trace_events.h:207:37: warning: ‘trace_event_type_funcs_ifs_status’ defined but not used [-Wunused-variable]
>   207 | static struct trace_event_functions trace_event_type_funcs_##call = {   \
>       |                                     ^~~~~~~~~~~~~~~~~~~~~~~
> /home/agluck/GIT/mywork/include/trace/events/intel_ifs.h:11:1: note: in expansion of macro ‘DECLARE_EVENT_CLASS’
>    11 | DECLARE_EVENT_CLASS(ifs_status,
>       | ^~~~~~~~~~~~~~~~~~~
> make[1]: Leaving directory '/home/agluck/GIT/mywork/build/ifsv5-rc1'
> 

Yeah, because you don't have more than one event, so the
DEFINE_EVENT_PRINT() does not make sense.  You still need one
DEFINE_EVENT() otherwise, you will get that static function not used
warning.

-- Steve