On Wed, Jun 25, 2025 at 08:25:52AM -0400, Mathieu Desnoyers wrote:
> 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.f...@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.desnoy...@efficios.com/
>
shazptr is more close to the general hazptr I proposed earlier:
https://lore.kernel.org/lkml/20240917143402.930114-1-boqun.f...@gmail.com/
, it's aimed as a global facility therefore no hazptr_domain is needed,
plus it supports non-busy waiting synchronize_shazptr() at the
beginning.
> and whether this helper against compiler optimizations would still be needed
> here:
>
> https://lore.kernel.org/lkml/20241008135034.1982519-2-mathieu.desnoy...@efficios.com/
>
For the current user, no, but eventually we are going to need it.
Regards,
Boqun
> 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
