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

Boqun Feng
2025-04-14T06:01:20Z

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

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2.47.1