linux-kernel - Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers

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Message-ID: <aUSfu4V9mrD7BQGl@tardis-2.local>
Date: Fri, 19 Dec 2025 09:43:39 +0900
From: Boqun Feng <boqun.feng@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Joel Fernandes <joel@...lfernandes.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,	linux-kernel@...r.kernel.org,
 Nicholas Piggin <npiggin@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Will Deacon <will@...nel.org>,	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>,	John Stultz <jstultz@...gle.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, Ingo Molnar <mingo@...hat.com>,
	Waiman Long <longman@...hat.com>,	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,	Vlastimil Babka <vbabka@...e.cz>,
 maged.michael@...il.com,	Mateusz Guzik <mjguzik@...il.com>,
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,	rcu@...r.kernel.org,
 linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers

On Thu, Dec 18, 2025 at 12:35:18PM -0500, Mathieu Desnoyers wrote:
[...]
> > Could you utilize this[1] to see a
> > comparison of the reader-side performance against RCU/SRCU?
> 
> Good point ! Let's see.
> 
> On a AMD 2x EPYC 9654 96-Core Processor with 192 cores,
> hyperthreading disabled,
> CONFIG_PREEMPT=y,
> CONFIG_PREEMPT_RCU=y,
> CONFIG_PREEMPT_HAZPTR=y.
> 
> scale_type                 ns
> -----------------------
> hazptr-smp-mb             13.1   <- this implementation
> hazptr-barrier            11.5   <- replace smp_mb() on acquire with barrier(), requires IPIs on synchronize.
> hazptr-smp-mb-hlist       12.7   <- replace per-task hp context and per-cpu overflow lists by hlist.
> rcu                       17.0

Hmm.. now looking back, how is it possible that hazptr is faster than
RCU on the reader-side? Because a grace period was happening and
triggered rcu_read_unlock_special()? This is actualy more interesting.

Regards,
Boqun

> srcu                      20.0
> srcu-fast                  1.5
> rcu-tasks                  0.0
> rcu-trace                  1.7
> refcnt                  1148.0
> rwlock                  1190.0
> rwsem                   4199.3
> lock                   41070.6
> lock-irq               46176.3
> acqrel                     1.1
> 
> So only srcu-fast, rcu-tasks, rcu-trace and a plain acqrel
> appear to beat hazptr read-side performance.
> 
> [...]
> 
[...]