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 972C5606A31B for ; Mon, 26 Oct 2026 09:05:14 +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=kqqcDEg5; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=Lf0VyYVn 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:Content-Type:MIME-Version: References:In-Reply-To:Subject:Cc:To:From:Message-ID: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=2aZQAb9b35wOHk5vXC8j1PTlTOhSerTxTS33wIhHbWs=; b=kqqcDEg5h0/1+NSH1N1XTXvEbV V96nfuYg2x/jp5OFhhoyq1d8IB9GKeC1mIpuT9OHTuoPCmPO8BJH0+bsUpLd6GkrjsP83X4OgoCYz 7ff2j/rflJt2V9fTbJ3HhPFD9tWq4N6m3MbiAqUPdm16p3gXPW+m7LBXK5O2hzqV7RtFjvybtul1m KdK9HjKrXbcYbWv6tRQbT2y9EP9ln8UaUAxEgUB+8BL8AMM1t+kMWf6OU8iHFWQLdvd+gNgBZA5D1 YYZvXuba9jnqln6oFKL8+PtfHVxAsrUAvbMrnALqWdedHeYgNxQ8RZGXmy4V+UqHVNBivMxnbMCz3 0/er3wYw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLGde-000000020gK-0wf7; Mon, 26 Oct 2026 09:05:06 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLGdd-000000020fz-3CAH for linux-arm-kernel@lists.infradead.org; Mon, 26 Oct 2026 09:05:05 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 342AA601DE; Mon, 26 Oct 2026 09:05:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DC6221F01825; Mon, 26 Oct 2026 09:05:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793005502; bh=2aZQAb9b35wOHk5vXC8j1PTlTOhSerTxTS33wIhHbWs=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=Lf0VyYVnAFTkLv4ZDT7e5V7Alx5II69sMEO3VoGqbaCxVmPvse+ZiYgyMqgIGTnKy YcNlVoPgiDUvPji1Y618fGb35FkCyvXBLf+u9EfuCp4sU8Ir2cUj/eFPbmN8ABiRes zWcrTK8WU/jiRb39RDKr9QygfB0i0yQZ7pAzxHLNfYI5PpUP730IoFWzwF13BPufiA gRXB3anc7GTHyIBy9ZnGEsKzC9y8O/qxpyesZ56Upi7KyrGHMflVKfgLbATVEJTRlO n5D/upAN1ZJR2WwwKbGUyaqJmtxfYOTeRgPHFo5rbdxj2yJ95XlpPaKu580F/10dJM kAndyV037Mthw== Received: from sofa.misterjones.org ([185.219.108.64] helo=goblin-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.98.2) (envelope-from ) id 1xLGdY-00000007B2X-3PVx; Mon, 26 Oct 2026 09:05:00 +0000 Date: Mon, 26 Oct 2026 09:05:00 +0000 Message-ID: <86zew0ygqr.wl-maz@kernel.org> From: Marc Zyngier To: zhangqiao22@huawei.com, Zhan Xusheng Cc: tglx@linutronix.de, jstultz@google.com, sboyd@kernel.org, peterz@infradead.org, mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, zhanxusheng@xiaomi.com Subject: Re: [QUESTION] sched/clock: Potential timestamp jump due to inter-CPU counter skew? In-Reply-To: <20261026082730.2117173-1-zhanxusheng1024@gmail.com> References: <20261026082730.2117173-1-zhanxusheng1024@gmail.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/30.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) X-Archived-By: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: zhangqiao22@huawei.com, zhanxusheng1024@gmail.com, tglx@linutronix.de, jstultz@google.com, sboyd@kernel.org, peterz@infradead.org, mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, zhanxusheng@xiaomi.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false 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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no 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 Mon, 26 Oct 2026 08:27:30 +0000, Zhan Xusheng wrote: > > On arm64 the system counter is meant to give all PEs a common view of > time, and keeping the copies on separate dies in step is the > implementation's job. The kernel has no tolerance for skew. The counter > errata handled in arm_arch_timer.c (Freescale A-008585, HiSilicon > 161010101, Cortex-A73 858921, Allwinner A64) are all reads on a single > CPU returning a wrong value, worked around by reading twice; none of them > is skew between CPUs. Indeed. If the implementation cannot give any two arbitrary CPUs anywhere in the system the exact same view of the counter, then this implementation is broken, and unsupported. No discussion. M. -- Without deviation from the norm, progress is not possible.