[RFC PATCH 0/8] Introduce simple hazard pointers for lockdep

[Boqun Feng <boqun.feng@gmail.com>]

Hi,

This RFC is mostly a follow-up on discussion:

	https://lore.kernel.org/lkml/20250321-lockdep-v1-1-78b732d195fb@debian.org/

I found that using a hazard pointer variant can speed up the
lockdep_unregister_key(), on my system (a 96-cpu VMs), the results of:

	time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq

are

	(without the patchset)
	real    0m1.039s
	user    0m0.001s
	sys     0m0.069s

	(with the patchset)
	real    0m0.053s
	user    0m0.000s
	sys     0m0.051s

i.e. almost 20x speed-up.

Other comparisons between RCU and shazptr, the rcuscale results (using
default configuration from
tools/testing/selftests/rcutorture/bin/kvm.sh):

RCU:

	Average grace-period duration: 7470.02 microseconds
	Minimum grace-period duration: 3981.6
	50th percentile grace-period duration: 6002.73
	90th percentile grace-period duration: 7008.93
	99th percentile grace-period duration: 10015
	Maximum grace-period duration: 142228

shazptr:

	Average grace-period duration: 0.845825 microseconds
	Minimum grace-period duration: 0.199
	50th percentile grace-period duration: 0.585
	90th percentile grace-period duration: 1.656
	99th percentile grace-period duration: 3.872
	Maximum grace-period duration: 3049.05

shazptr (skip_synchronize_self_scan=1, i.e. always let scan kthread to
wakeup):

	Average grace-period duration: 467.861 microseconds
	Minimum grace-period duration: 92.913
	50th percentile grace-period duration: 440.691
	90th percentile grace-period duration: 460.623
	99th percentile grace-period duration: 650.068
	Maximum grace-period duration: 5775.46

shazptr_wildcard (i.e. readers always use SHAZPTR_WILDCARD):

	Average grace-period duration: 599.569 microseconds
	Minimum grace-period duration: 1.432
	50th percentile grace-period duration: 582.631
	90th percentile grace-period duration: 781.704
	99th percentile grace-period duration: 1160.26
	Maximum grace-period duration: 6727.53

shazptr_wildcard (skip_synchronize_self_scan=1):

	Average grace-period duration: 460.466 microseconds
	Minimum grace-period duration: 303.546
	50th percentile grace-period duration: 424.334
	90th percentile grace-period duration: 482.637
	99th percentile grace-period duration: 600.214
	Maximum grace-period duration: 4126.94
	

Overall it looks promising to me, but I would like to see how it
performs in the environment of Breno. Also as Paul always reminds me:
buggy code usually run faster, so please take a look in case I'm missing
something ;-) Thanks!

The patchset is based on v6.15-rc1.

Boqun Feng (8):
  Introduce simple hazard pointers
  shazptr: Add refscale test
  shazptr: Add refscale test for wildcard
  shazptr: Avoid synchronize_shaptr() busy waiting
  shazptr: Allow skip self scan in synchronize_shaptr()
  rcuscale: Allow rcu_scale_ops::get_gp_seq to be NULL
  rcuscale: Add tests for simple hazard pointers
  locking/lockdep: Use shazptr to protect the key hashlist

 include/linux/shazptr.h  |  73 +++++++++
 kernel/locking/Makefile  |   2 +-
 kernel/locking/lockdep.c |  11 +-
 kernel/locking/shazptr.c | 318 +++++++++++++++++++++++++++++++++++++++
 kernel/rcu/rcuscale.c    |  60 +++++++-
 kernel/rcu/refscale.c    |  77 ++++++++++
 6 files changed, 534 insertions(+), 7 deletions(-)
 create mode 100644 include/linux/shazptr.h
 create mode 100644 kernel/locking/shazptr.c

-- 
2.47.1