[PATCH v9 0/7] Add support for length of each segment in the segcblist

[Joel Fernandes (Google)]

This is required for several usecases identified. One of them being tracing how
the segmented callback list changes. Tracing this has identified issues in RCU
code in the past.

>From Paul:
Another use case is of course more accurately determining whether a given CPU's
large pile of callbacks can be best served by making grace periods go faster,
invoking callbacks more vigorously, or both.  It should also be possible to
simplify some of the callback handling a bit, given that some of the unnatural
acts are due to there having been no per-batch counts.

Revision history:
v9: Fix SRCU issues, other minor style changes (Frederic). Added Frederic's
Reviewed-by to all but the last patch..

v8: Small style changes, making the seglen as non-atomic since it is always
    under lock (Frederic).

v7: Cleaned up memory barriers (thanks fweisbec@ for reviewing), made minor
corrections per Neeraj (thanks).

v6: Fixed TREE04, and restored older logic to ensure rcu_barrier works.

v5: Various changes, bug fixes. Discovery of rcu_barrier issue.

v4: Restructured rcu_do_batch() and segcblist merging to avoid issues.
    Fixed minor nit from Davidlohr.
v1->v3: minor nits.
(https://lore.kernel.org/lkml/20200719034210.2382053-1-joel@xxxxxxxxxxxxxxxxx/)

Joel Fernandes (Google) (7):
rcu/tree: Make rcu_do_batch count how many callbacks were executed
rcu/segcblist: Add counters to segcblist datastructure
srcu: Fix invoke_rcu_callbacks() segcb length adjustment
rcu/trace: Add tracing for how segcb list changes
rcu/segcblist: Remove useless rcupdate.h include
rcu/tree: segcblist: Remove redundant smp_mb()s
rcu/segcblist: Add additional comments to explain smp_mb()

include/linux/rcu_segcblist.h |   1 +
include/trace/events/rcu.h    |  25 +++++
kernel/rcu/rcu_segcblist.c    | 198 +++++++++++++++++++++++++---------
kernel/rcu/rcu_segcblist.h    |   8 +-
kernel/rcu/srcutree.c         |   5 +-
kernel/rcu/tree.c             |  21 ++--
6 files changed, 199 insertions(+), 59 deletions(-)

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2.29.1.341.ge80a0c044ae-goog