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

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Message-ID: <20251106110907.noLpnulw@linutronix.de>
Date: Thu, 6 Nov 2025 12:09:07 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
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>
Subject: Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting

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.

> 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