linux-kernel - Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting

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Message-ID: <20251106112339.GQ3245006@noisy.programming.kicks-ass.net>
Date: Thu, 6 Nov 2025 12:23:39 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting

On Thu, Nov 06, 2025 at 12:09:07PM +0100, Sebastian Andrzej Siewior wrote:
> On 2025-11-06 10:29:29 [+0100], Peter Zijlstra wrote:
> > Subject: futex: Optimize per-cpu reference counting
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Wed, 16 Jul 2025 16:29:46 +0200
> > 
> > Shrikanth noted that the per-cpu reference counter was still some 10%
> > slower than the old immutable option (which removes the reference
> > counting entirely).
> > 
> > Further optimize the per-cpu reference counter by:
> > 
> >  - switching from RCU to preempt;
> >  - using __this_cpu_*() since we now have preempt disabled;
> >  - switching from smp_load_acquire() to READ_ONCE().
> > 
> > This is all safe because disabling preemption inhibits the RCU grace
> > period exactly like rcu_read_lock().
> > 
> > Having preemption disabled allows using __this_cpu_*() provided the
> > only access to the variable is in task context -- which is the case
> > here.
> 
> Right. Read and Write from softirq happens after the user transitioned
> to atomics.
> 
> > Furthermore, since we know changing fph->state to FR_ATOMIC demands a
> > full RCU grace period we can rely on the implied smp_mb() from that to
> > replace the acquire barrier().
> 
> That is the only part I struggle with but having a smp_mb() after a
> grace period sounds reasonable.

IIRC the argument goes something like so:

A grace-period (for rcu-sched, which is implied by regular rcu)
implies that every task has done at least one voluntary context switch.
A context switch implies a full barrier.

Therefore observing a state change separated by a grace-period implies
an smp_mb().

> > This is very similar to the percpu_down_read_internal() fast-path.
> >
> > The reason this is significant for PowerPC is that it uses the generic
> > this_cpu_*() implementation which relies on local_irq_disable() (the
> > x86 implementation relies on it being a single memop instruction to be
> > IRQ-safe). Switching to preempt_disable() and __this_cpu*() avoids
> > this IRQ state swizzling. Also, PowerPC needs LWSYNC for the ACQUIRE
> > barrier, not having to use explicit barriers safes a bunch.
> > 
> > Combined this reduces the performance gap by half, down to some 5%.
> 
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> Sebastian