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

[Paul E. McKenney]

Hello!

This v3 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, simplifies the code, 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.

15.	Warn if ->interval or ->burst are negative, courtesy of Petr
	Mladek.

16.	Simplify common-case exit path.

17.	Use nolock_ret label to save a couple of lines of code.

18.	Use nolock_ret label to collapse lock-failure code.

19.	Use nolock_ret restructuring to collapse common case code.

20.	Drop redundant accesses to burst.

Changes since v2:

o	Apply feedback from Bert Karwatzki, Srikanth Aithal, and Mark
	Brown, fixing a hang that happened on some systems.

o	Applied Reviewed-by tags and added links.

o	Added a prototype patch from Petr Mladek that splats if either
	interval or burst are negative.

o	Added several commits that simplify the code.

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/ratelimit.c                    |    8 -
 b/lib/tests/Makefile                 |    1 
 b/lib/tests/test_ratelimit.c         |   79 ++++++++++++++++
 include/linux/ratelimit.h            |   13 +-
 include/linux/ratelimit_types.h      |    3 
 lib/ratelimit.c                      |  165 ++++++++++++++++++++---------------
 12 files changed, 246 insertions(+), 104 deletions(-)