[RFC PATCH v4 0/7] fix task state report from sched tracepoint

[Ze Gao]

This is the 4th attempt to fix the report task state issue in sched
tracepint, you can check out previous discussions here:

v1: https://lore.kernel.org/linux-trace-kernel/20230725072254.32045-1-zegao@xxxxxxxxxxx
v2: https://lore.kernel.org/linux-trace-kernel/20230726121618.19198-1-zegao@xxxxxxxxxxx
v3: https://lore.kernel.org/linux-trace-kernel/20230801090124.8050-1-zegao@xxxxxxxxxxx

Against v3, reorganize commits so that they can be cherry-picked
individually if people indeed have strong opinion over this change.

Note PATCH 1-4 are the normal fixes and cleanup whereas PATCH 5-7
introduce new changes.

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FYI, this series are designed not to break anything now and still do the 
1-1 correspondence int-char mapping for each distinct task state we want to
report, and thus will not lose any details intended for debug purposes. Of
course, this might be compromised because of bugs introduced due to my
stupidity. So your sage comments are very important and appreciated!

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In the status quo, we should see three different outcomes of the reported
sched-out task state from perf-script, perf-sched-timehist, and Tp_printk
of tracepoint sched_switch.  And it's not hard to figure out that the
former two are built upon the third one, and the reason why we see this
inconsistency is that the former two does not catch up with the internal
change of reported task state definitions as the kernel evolves.

IMHO, exporting internal representations of task state in the tracepoint
sched_switch is not a good practice and not encouraged at all, which can
easily break userspace tools that relies on it. Especially when tracepoints
are massively used in many observability tools nowadays due to its stable
nature, which makes them no longer used for debug only purpose and we
should be careful to decide what ought to be reported to userspace and what
ought not.

Therefore, to fix the issues mentioned above for good, I proposed to add
a new variable to report task state in sched_switch with a symbolic char
along with the old hardcoded value, and save the further processing of
userspace tools and spare them from knowing implementation details in the
kernel.

After this patch seires, we report 'RSDTtXZPI' the same as in procfs, plus
a 'p' which denotes PREEMP_ACTIVE and is used for sched_switch tracepoint
only.

Reviews welcome!

Regards,
Ze

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Ze Gao (5):
  perf sched: sync state char array with the kernel
  perf sched: reorganize sched-out task state report code
  sched, tracing: reorganize fields of switch event struct
  sched, tracing: add to report task state in symbolic chars
  perf sched: prefer to use prev_state_char introduced in sched_switch

 include/trace/events/sched.h | 68 +++++++++++++++++-------------
 tools/perf/builtin-sched.c   | 82 ++++++++++++++++--------------------
 2 files changed, 76 insertions(+), 74 deletions(-)

Ze Gao (2):
  libtraceevent: sync state char array with the kernel
  libtraceevent: prefer to use prev_state_char introduced in
    sched_switch

 plugins/plugin_sched_switch.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

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2.41.0