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

["Luck, Tony" <tony.luck@intel.com>]

On Mon, Apr 25, 2022 at 09:49:28PM -0400, Steven Rostedt wrote:
> On Mon, 25 Apr 2022 16:49:35 +0000
> "Luck, Tony" <tony.luck@intel.com> wrote:
> 
> > I see two paths:
> > 
> > 1) Create a new user friendly trace point for each new scan mode.
> > 2) Just provide a generic one that dumps both the 64-bit WRMSR and RDMSR values.
> > 
> > Q: Are trace points "expensive" in some way ... so better to just have one than three?
> >      Or are the cheap enough that decoding for the user is an OK thing?
> 
> Yes, they are expensive as each TRACE_EVENT() can add a few KB of text and
> data. But you can add a DECLARE_EVENT_CLASS() and then add "printk"
> differences that are less memory heavy.
> 
> See DEFINE_EVENT_PRINT().

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.

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'

-Tony