From nobody Wed Oct 8 08:20:19 2025 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 392D128B7F3 for ; Mon, 30 Jun 2025 14:50:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751295039; cv=none; b=LDKvodliLCthGKZ44+iQHcZzRRuHhPrLm6SKI3TmcVeybgMrHTYQEPfrAF6G+JF0DlrOp5LqOXcrmdULp0mZ8oRvkDKE5fvJyRSSXbjA+oxbUY4rGvk2jzCQoPJWYGlYqi6tN+y0JZ07vKVcZPfXmfEnC9xk56UrKd4RaBevbhE= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751295039; c=relaxed/simple; bh=JKcdzskNBG2gv+2opYysBC76J4fT6gAvUPmpMbNQ7n0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=k5KofiYSVEjmdPoDZTwXy0kwXHl33L0K/k1POeFMXBCanb+cOe9yoUy7pkQ6AGFJ6A9GCZnCnxYTgcnrqH8vZ4gmCz7hloWBF8RcPgNY7BdNFDhoUAEgGJ+SdVFDHV1hD2pFMeV2foM9qS8OhZ3AHn2eo/W9JW7WC9JDCOQq2b4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=tyC1qnPE; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=ag/1SJFq; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="tyC1qnPE"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="ag/1SJFq" Date: Mon, 30 Jun 2025 16:50:34 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1751295035; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=AyTl6LkN47yVn0x7QRLEVXaJXcJVffHJPLhqBWqczyY=; b=tyC1qnPEnUq6FG4isqzrMAfH/wDt2soQaH1NmNql9xzngbUiffF3QWmak8d+fgkKrfDDFI vL5rZ9uMa2nQgDc4Nwlr/hYI2h9GGZ31VikowNJv46IY+pUM1PibpbwaNVZSXxg2Iy6JL/ pYF9Ys87zxWYSy7H/V8QJYaNUSj/mw5IgYFL2Xy3y5fHVAHGRYEUXOd+QwMsbSSIWE/dhl JLnkurhpfw647y6c0YKEOWtt+awBlVsA/WaQBoO86RHDJSzb/3mscUVvwvaPUnLMuNrBEe 9k/AXky7WdFQa08Q4W5c++ewhMjJ1XuSaYnl6jSDz5zkleoPZLdgMHiiGljVLw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1751295035; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=AyTl6LkN47yVn0x7QRLEVXaJXcJVffHJPLhqBWqczyY=; b=ag/1SJFqsRjy+8iChnMvR+ls3XAP2JyWURmL3GNw/HQ/qqGknO6k22rtXavy6BzrgV0R9y fLDYYZOat0KEB6Cw== From: Sebastian Andrzej Siewior To: Peter Zijlstra Cc: Chris Mason , linux-kernel@vger.kernel.org, Thomas Gleixner Subject: [PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH Message-ID: <20250630145034.8JnINEaS@linutronix.de> References: <20250604092815.UtG-oi0v@linutronix.de> <372d8277-7edb-4f78-99bd-6d23b8f94984@meta.com> <20250604200808.hqaWJdCo@linutronix.de> <20250606070638.2Wk45AMk@linutronix.de> <20250624190118.GB1490279@noisy.programming.kicks-ass.net> <71ea52f2-f6bf-4a55-84ba-d1442d13bc82@meta.com> <20250626131715.GI1613200@noisy.programming.kicks-ass.net> <20250626135034.18sab4Xp@linutronix.de> <20250627110457.GA1613200@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20250627110457.GA1613200@noisy.programming.kicks-ass.net> Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Chris Mason reported a performance regression on big iron. Reports of this kind were usually reported as part of a micro benchmark but Chris' test did mimic his real workload. This makes it a real regression. The root cause is rcuref_get() which is invoked during each futex operation. If all threads of an application do this simultaneously then it leads to cache line bouncing and the performance drops. Disable FUTEX_PRIVATE_HASH entirely for this cycle. The performance regression will be addressed in the following cycle enabling the option again. Reported-by: Chris Mason Closes: https://lore.kernel.org/all/3ad05298-351e-4d61-9972-ca45a0a50e33@me= ta.com/ Signed-off-by: Sebastian Andrzej Siewior --- init/Kconfig | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/init/Kconfig b/init/Kconfig index af4c2f0854554..666783eb50abd 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1716,9 +1716,13 @@ config FUTEX_PI depends on FUTEX && RT_MUTEXES default y =20 +# +# marked broken for performance reasons; gives us one more cycle to sort t= hings out. +# config FUTEX_PRIVATE_HASH bool depends on FUTEX && !BASE_SMALL && MMU + depends on BROKEN default y =20 config FUTEX_MPOL --=20 2.50.0