[PATCH v3 0/2] taskstats: fix TGID dead-thread stat retention

[Yiyang Chen <cyyzero16@gmail.com>]

This series fixes a taskstats TGID aggregation bug where fields added in
the TGID query path were not preserved after thread exit, and adds a
kselftest covering the regression.

The first patch keeps the cached TGID aggregate used for dead threads in
step with the fields already accumulated for live threads, and also fixes
the final TGID exit notification emitted when group_dead is true.

The second patch adds a kselftest that verifies TGID CPU stats do not
regress after a worker thread exits and has been reaped.

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Changes in v3:
- fix the helper comment in patch 1 to use per-task-foo(stats, tsk)
- add Acked-by tags to both patches
- Link to v2: <https://lore.kernel.org/all/cover.1776020234.git.cyyzero16@gmail.com/>

Changes in v2:
- add Fixes tags for the two commits that introduced the regression
- clarify that the regression affects both TGID queries and the final
  TGID exit notification
- add a kselftest that checks TGID CPU stats do not regress after
  thread exit
- Link to v1: <https://lore.kernel.org/all/6f4ed79d96c389a9a1d67d5ced96c6326eda82ae.1774552296.git.cyyzero16@gmail.com/>

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2.43.0