From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A5AB1606A31A for ; Tue, 27 Oct 2026 12:00:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=W0sv9jGI; 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=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Unsubscribe-Post:List-Owner; bh=22OilN3OWHAsf4ZnKYCUCqyoE4jo68G/Rnat5EPiPAs=; b=W0sv9jGIu4inYY+TpMg1TGNGD2 kUTsXszIV79QpJApiqLzfyKOatj9Sv01uK/eTa0Hm4qnlYvooxKE2FYZUSuem61dPmdmZBNl5lnXq uIzw7leokmR/T2Anw0vCKDUsZNOpeUQDYH0kzISyvAirIMnh5xdKG7LmDGLVE284iMRMn9h/53gB2 gDeon18WvOU+ptzqFtSoJNmj2K+teDPe40olpLQ9I8BQGqWIwLt0D2PgSt8xB0L2Nb0H5d+EdRqtg d43EUWsPZKRpCyzJetZdck09Lgg/G6A4vLIJK3s2ba5Lw2ohoyPjDHo/mw26QvN0TRUPBOeC1E9Ub ZE4govpQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLfqd-00000004Xvt-3rKn; Tue, 27 Oct 2026 12:00:11 +0000 Received: from stravinsky.debian.org ([2001:41b8:202:deb::311:108]) by bombadil.infradead.org with esmtps (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLfqa-00000004Xv8-1h84 for linux-arm-kernel@lists.infradead.org; Tue, 27 Oct 2026 12:00:10 +0000 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-Archived-By: 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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261027_050008_454101_1151E62B X-CRM114-Status: GOOD ( 17.67 ) X-Spam-Score: -4.4 (----) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-16.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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?