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Message-ID: <aFv31IIAS0fiYci5@Mac.home> Date: Wed, 25 Jun 2025 06:21:24 -0700 From: Boqun Feng <boqun.feng@...il.com> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, lkmm@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Davidlohr Bueso <dave@...olabs.net>, "Paul E. McKenney" <paulmck@...nel.org>, Josh Triplett <josh@...htriplett.org>, Frederic Weisbecker <frederic@...nel.org>, Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Joel Fernandes <joelagnelf@...dia.com>, Uladzislau Rezki <urezki@...il.com>, Steven Rostedt <rostedt@...dmis.org>, Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>, Breno Leitao <leitao@...ian.org>, aeh@...a.com, netdev@...r.kernel.org, edumazet@...gle.com, jhs@...atatu.com, kernel-team@...a.com, Erik Lundgren <elundgren@...a.com> Subject: Re: [PATCH 0/8] Introduce simple hazard pointers for lockdep 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.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/ > shazptr is more close to the general hazptr I proposed earlier: https://lore.kernel.org/lkml/20240917143402.930114-1-boqun.feng@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.desnoyers@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
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