Re: [PATCH RFC 0/9] Reduce ratelimit's false-positive misses

[Paul E. McKenney]

Hello!

This v2 series replaces open-coded uses of the ratelimit_state structure
with formal APIs, counts all rate-limit misses, replaces jiffies=0 special
case with a flag, provides a ___ratelimit() trylock-failure fastpath
to (almost) eliminate false-positive misses, and adds a simple test.

The key point of this series is the reduction of false-positive misses.

The individual patches are as follows:

1.	Add trivial kunit test for ratelimit.

2.	Create functions to handle ratelimit_state internals.

3.	Avoid open-coded use of ratelimit_state structure's ->missed
	field.

4.	Avoid open-coded use of ratelimit_state structure's ->missed
	field.

5.	Avoid open-coded use of ratelimit_state structure's internals.

6.	Convert the ->missed field to atomic_t.

7.	Count misses due to lock contention.

8.	Avoid jiffies=0 special case.

9.	Reduce ___ratelimit() false-positive rate limiting, courtesy of
	Petr Mladek.

10.	Allow zero ->burst to disable ratelimiting.

11.	Force re-initialization when rate-limiting re-enabled.

12.	Don't flush misses counter if RATELIMIT_MSG_ON_RELEASE.

13.	Avoid atomic decrement if already rate-limited.

14.	Avoid atomic decrement under lock if already rate-limited.

Changes since v1 (RFC):

o	Switch from lockless fastpath to carrying out needed updates
	upon trylock failure, per Petr Mladek feedback.  This greatly
	simplifies the code and is a much smaller change from the
	current code.  There is a small performance penalty compared to
	the lockless fastpath, but not enough to matter.

o	Never unconditionally acquire the lock, again per Petr Mladek
	feedback.

o	Better define effects of non-positive burst values (always
	ratelimit) and non-positive interval values (never ratelimit
	when the burst value is positive).

o	The changes from Petr's original are supplied as five incremental
	patches, but could easily be folded into Petr's original if
	desired.  (Left to my lazy self, they stay as-is.)

						Thanx, Paul

------------------------------------------------------------------------

 b/drivers/char/random.c              |    9 +--
 b/drivers/gpu/drm/amd/pm/amdgpu_pm.c |   11 ---
 b/drivers/gpu/drm/i915/i915_perf.c   |    8 +-
 b/include/linux/ratelimit.h          |   40 ++++++++++++-
 b/include/linux/ratelimit_types.h    |    2 
 b/lib/Kconfig.debug                  |   11 +++
 b/lib/Makefile                       |    1 
 b/lib/ratelimit.c                    |    8 +-
 b/lib/test_ratelimit.c               |   79 +++++++++++++++++++++++++++
 include/linux/ratelimit.h            |   13 +---
 include/linux/ratelimit_types.h      |    3 -
 lib/ratelimit.c                      |  102 +++++++++++++++++++++++++----------
 12 files changed, 223 insertions(+), 64 deletions(-)