linux-kernel - Re: futex performance regression from "futex: Allow automatic allocation of process wide futex hash"

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Message-ID: <20250627110457.GA1613200@noisy.programming.kicks-ass.net>
Date: Fri, 27 Jun 2025 13:04:57 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Chris Mason <clm@...a.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: futex performance regression from "futex: Allow automatic
 allocation of process wide futex hash"

On Thu, Jun 26, 2025 at 03:50:34PM +0200, Sebastian Andrzej Siewior wrote:
> On 2025-06-26 15:17:15 [+0200], Peter Zijlstra wrote:
> > > I'd vote for defaulting to global for one more release.
> > 
> > Probably best to do that; means we don't have to rush crazy code :-)
> 
> If we do that, that means we need to ensure that the private hash is not
> deployed while a thread is created and the prctl() function can only
> work create the private hash if the application is single threaded.

I was thinking something like so..

---
diff --git a/init/Kconfig b/init/Kconfig
index af4c2f085455..666783eb50ab 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1716,9 +1716,13 @@ config FUTEX_PI
 	depends on FUTEX && RT_MUTEXES
 	default y
 
+#
+# marked broken for performance reasons; gives us one more cycle to sort things out.
+#
 config FUTEX_PRIVATE_HASH
 	bool
 	depends on FUTEX && !BASE_SMALL && MMU
+	depends on BROKEN
 	default y
 
 config FUTEX_MPOL