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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-06-24 23:10, Boqun Feng wrote:
> Hi,
> 
> This is the official first version of simple hazard pointers following
> the RFC:
> 
> 	https://lore.kernel.org/lkml/20250414060055.341516-1-boqun.feng@gmail.com/
> 
> I rebase it onto v6.16-rc3 and hope to get more feedback this time.
> 
> Thanks a lot for Breno Leitao to try the RFC out and share the numbers.
> 
> I did an extra comparison this time, between the shazptr solution and
> the synchronize_rcu_expedited() solution. In my test, during a 100 times
> "tc qdisc replace" run:
> 
> * IPI rate with the shazptr solution: ~14 per second per core.
> * IPI rate with synchronize_rcu_expedited(): ~140 per second per core.
> 
> (IPI results were from the 'CAL' line in /proc/interrupt)
> 
> This shows that while both solutions have the similar speedup, shazptr
> solution avoids the introduce of high IPI rate compared to
> synchronize_rcu_expedited().
> 
> Feedback is welcome and please let know if there is any concern or
> suggestion. Thanks!

Hi Boqun,

What is unclear to me is what is the delta wrt:

https://lore.kernel.org/lkml/20241008135034.1982519-4-mathieu.desnoyers@efficios.com/

and whether this helper against compiler optimizations would still be needed here:

https://lore.kernel.org/lkml/20241008135034.1982519-2-mathieu.desnoyers@efficios.com/

Thanks,

Mathieu

> 
> Regards,
> Boqun
> 
> --------------------------------------
> Please find the old performance below:
> 
> On my system (a 96-cpu VMs), the results of:
> 
> 	time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
> 
> are (with lockdep enabled):
> 
> 	(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
> 
> 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
> 


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com