From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=82.195.75.108 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793102406; b=vcKiQDB8Wprg9sFCsIKz3Z+Bb3mVBglgH1FmigqOuZbzZcnXwsV0KTeZ0KXZUkQP6nXG YH0+Q/ho7os4m1LpDtFq2nhpuGEMfzzCcQgeqo0LIC3FT1h20WcEMM+rv2Wb53zTVCSsj 0l6wACpa6eTNcq3hY56xZ3CM0hdpKwYaWDAOEtQSCsKIN05mb3CfLWL7eEuP8jtXraA3m 5zkp59q0LCHM9qp1nr/kEM4dk8FNnxP/a12zWmeuBi9NNNWjAbjEUWxFkk7mkGfGMOBhN gboDsJE6j1IL0KrLzUPNqnu+kbuwCFfm9eL9dByiqYjH1f+84ez/mRzIAyY45uY0egA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793102406; h=DMARC-Filter:DKIM-Signature:Received:Date:From:To:Cc:Subject: Message-ID:References:MIME-Version:Content-Type:Content-Disposition: In-Reply-To:X-Debian-User; bh=22OilN3OWHAsf4ZnKYCUCqyoE4jo68G/Rnat5EPiPAs=; b=TgRoBw+DD/Zz6uy/0Yq9qz+s5xkSc4RxCMsR7sSSFC6rVDnyCI7mDdolrqjyPPNQ6cro 1/qjehp9bEB/9NNcs4CRxVYjMa3PV7ZASutau8PVvVJEb7m/rHG7GSUQ5LCeDga07qugv bM66XPUYOaETabPbmXl8njO9mYFi7n7/U5ly1Gkl3OTc2soWUKTuDt7SI1ZIRlnqMqGFW Ka1n00qvZuXdKPRhSbnC9zY4Jb9UG3deCb8PNgyWx5JRFkn+NpOap/dVON+kpsLZpmjFf GdD8o66dDqqLCgQdTf4kCJff8Ms2XjgUFD81JdSOGRtFpl9MNwN27JixJFwXm9ok8qw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=debian.org; spf=pass smtp.mailfrom=debian.org; dkim=pass header.d=debian.org header.i=@debian.org header.a=rsa-sha256 header.s=smtpauto.stravinsky header.b="hZ/Ow0ro"; arc=none smtp.remote-ip=82.195.75.108 Received: from stravinsky.debian.org (stravinsky.debian.org [82.195.75.108]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (prime256v1) server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7DB5C606A31D for ; Tue, 27 Oct 2026 12:00:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 7DB5C606A31D Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=debian.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=debian.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=debian.org header.i=@debian.org header.a=rsa-sha256 header.s=smtpauto.stravinsky header.b=hZ/Ow0ro DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=debian.org; s=smtpauto.stravinsky; h=X-Debian-User:In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=22OilN3OWHAsf4ZnKYCUCqyoE4jo68G/Rnat5EPiPAs=; b=hZ/Ow0roMzSE6yS/Enuvr71b/R dPnK8+fdUYq9MNWyNw3mFA9CBhIs0m1St9Yk2Ddl1mOzI0ghUTkS5l0LOyAgA489QRwVzkXufKIF/ tRzXxd419Nq5fXvU35Yr6bMft+1/iuV3bYQ0gH7kOQZTa17H/fO2V7fckLIlE8ZfqQgYLxI/6Gw1Z wv8/3NpmC2nyEaPvJRvmLMCXtMFPkjBH2xJrZgLNn1K68bkfU0CidLRudY0jFLqozzH19UywHckQp 4YqXz43UaUzygXk5AXuyPMku/yNa8IvggNT+rHYiZH10lYxKZqb+smsAoShzC9bYA4lngzg/WN35f BG04PZrw==; Received: from authenticated-user by stravinsky.debian.org with esmtpsa (TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.96) (envelope-from ) id 1xLfqP-001RTE-0X; Tue, 27 Oct 2026 11:59:57 +0000 Date: Tue, 27 Oct 2026 04:59:52 -0700 From: Breno Leitao To: Catalin Marinas , Will Deacon , Mark Rutland Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kas@kernel.org, kernel-team@meta.com Subject: Re: [PATCH] arm64: Kconfig: Don't select CPUMASK_OFFSTACK with 64K pages Message-ID: References: <20261007-arm_stack-v1-1-b48dad5fa8b2@debian.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261007-arm_stack-v1-1-b48dad5fa8b2@debian.org> X-Debian-User: leitao On Wed, Oct 07, 2026 at 12:58:16AM -0700, Breno Leitao wrote: > arm64 selects CPUMASK_OFFSTACK when NR_CPUS is above 256, which keeps > cpumask_var_t off the kernel stack. With 64K pages the kernel stack is > 64K as well, we have enough room to keep cpumask on the stack, avoiding > extra alocation and possible performance penalties. > > Skip the select when ARM64_64K_PAGES is set. A 64K kernel can then raise > NR_CPUS above 256 and keep cpumask_var_t as an inline array. 4K and 16K > page kernels are unchanged. > > Signed-off-by: Breno Leitao > --- > Based on Will presentation at LPC, I've decided to have a look at Meta's > stack usage, and I found that this might be an small optimization!? > --- > arch/arm64/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index 5f2db2cecf9772..b73371327d2a47 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -111,7 +111,7 @@ config ARM64 > select CLONE_BACKWARDS > select COMMON_CLK > select CPU_PM if (SUSPEND || CPU_IDLE) > - select CPUMASK_OFFSTACK if NR_CPUS > 256 > + select CPUMASK_OFFSTACK if NR_CPUS > 256 && !ARM64_64K_PAGES Does this one sound sane?