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

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Message-ID: <20250716142946.GD905792@noisy.programming.kicks-ass.net>
Date: Wed, 16 Jul 2025 16:29:46 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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 Tue, Jul 15, 2025 at 10:34:24PM +0530, Shrikanth Hegde wrote:

> I did try again by going to baseline, removed BROKEN and ran below. Which gives us immutable numbers.
> ./perf bench futex hash -Ib512
> Averaged 1536035 operations/sec (+- 0.11%), total secs = 10
> Futex hashing: 512 hash buckets (immutable)
> 
> So, with -b 512 option, it is around 8-10% less compared to immutable.

Urgh, can you run perf on that and tell me if this is due to
this_cpu_{inc,dec}() doing local_irq_disable() or the smp_load_acquire()
doing LWSYNC ?

Anyway, I think we can improve both. Does the below help?


---
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index d9bb5567af0c..8c41d050bd1f 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1680,10 +1680,10 @@ static bool futex_ref_get(struct futex_private_hash *fph)
 {
 	struct mm_struct *mm = fph->mm;
 
-	guard(rcu)();
+	guard(preempt)();
 
-	if (smp_load_acquire(&fph->state) == FR_PERCPU) {
-		this_cpu_inc(*mm->futex_ref);
+	if (READ_ONCE(fph->state) == FR_PERCPU) {
+		__this_cpu_inc(*mm->futex_ref);
 		return true;
 	}
 
@@ -1694,10 +1694,10 @@ static bool futex_ref_put(struct futex_private_hash *fph)
 {
 	struct mm_struct *mm = fph->mm;
 
-	guard(rcu)();
+	guard(preempt)();
 
-	if (smp_load_acquire(&fph->state) == FR_PERCPU) {
-		this_cpu_dec(*mm->futex_ref);
+	if (READ_ONCE(fph->state) == FR_PERCPU) {
+		__this_cpu_dec(*mm->futex_ref);
 		return false;
 	}