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The first part[1] of the LWN series on using TRACE_EVENT() mentions
about TP_printk():
"Do not create new tracepoint-specific helpers, because that will
confuse user-space tools that know about the TRACE_EVENT() helper
macros but will not know how to handle ones created for individual
tracepoints."
It seems this is what we ended up doing when using pipe_name() in
TP_printk().
For example, the format for the intel_pipe_update_start event is as
follows:
# cat /sys/kernel/debug/tracing/events/i915/intel_pipe_update_start/format
name: intel_pipe_update_start
ID: 1136
format:
field:unsigned short common_type; offset:0; size:2; signed:0;
field:unsigned char common_flags; offset:2; size:1; signed:0;
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
field:int common_pid; offset:4; size:4; signed:1;
field:__data_loc char[] dev; offset:8; size:4; signed:0;
field:enum pipe pipe; offset:12; size:4; signed:1;
field:u32 frame; offset:16; size:4; signed:0;
field:u32 scanline; offset:20; size:4; signed:0;
field:u32 min; offset:24; size:4; signed:0;
field:u32 max; offset:28; size:4; signed:0;
print fmt: "dev %s, pipe %c, frame=%u, scanline=%u, min=%u, max=%u", __get_str(dev), ((REC->pipe) + 'A'), REC->frame, REC->scanline, REC->min, REC->max
The call to pipe_name(__entry->pipe) is expanted to ((REC->pipe) + 'A')
and that's how the format is saved.
Even though the output from /sys/kernel/debug/tracing/trace will look
correct (because it is generated in the kernel), we will see corrupted
lines when using a tool like trace-cmd to view the data.
While the output looks correct when looking at
/sys/kernel/debug/tracing/trace, we see corrupted lines when viewing the
trace data with "trace-cmd report":
$ trace-cmd report \
> | sed -n 's/.*dev 0000:00:02\.0, \(pipe .\).*/\1/p' \
> | cat -v | uniq -c
34 pipe ^A
, where ^A is a non-printable character.
As a fix, let's store the pipe name directly in the event. The fix was
done by applying the following sed script:
s/__field\s*(\s*enum\s\+pipe\s*,\s*pipe\s*)/__field(char, pipe_name)/
s/__entry\s*->\s*pipe\s*=\s*\([^;]\+\);/__entry->pipe_name = pipe_name(\1);/
s/pipe_name\s*(\s*__entry\s*->\s*pipe\s*)/__entry->pipe_name/
After these changes, using the same example, we have the following:
$ trace-cmd report \
> | sed -n 's/.*dev 0000:00:02\.0, \(pipe .\).*/\1/p' \
> | cat -v | sort | uniq -c
396 pipe A
34 pipe B
[1] https://lwn.net/Articles/379903/
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016135300.21428-4-gustavo.sousa@intel.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.