From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 17056CCF9F0 for ; Wed, 29 Oct 2025 14:53:06 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 675FE8E007C; Wed, 29 Oct 2025 10:53:05 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 64E648E0045; Wed, 29 Oct 2025 10:53:05 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5639A8E007C; Wed, 29 Oct 2025 10:53:05 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 41DEA8E0045 for ; Wed, 29 Oct 2025 10:53:05 -0400 (EDT) Received: from smtpin04.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id E06A9140814 for ; Wed, 29 Oct 2025 14:53:04 +0000 (UTC) X-FDA: 84051444288.04.F7E46DA Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf13.hostedemail.com (Postfix) with ESMTP id 44F3020007 for ; Wed, 29 Oct 2025 14:53:03 +0000 (UTC) Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=NbRDLF5b; spf=pass (imf13.hostedemail.com: domain of frederic@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=frederic@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1761749583; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=sVrVIr57b6fMWh3pA4LxJ9hZO8cVb5LaT8DX+ge7ENU=; b=E3ZN91Tuboj3WBjB58SCRgNu6TMkvyVgGs1b5e6IFiVeDamu0bNUW8MD0uDD0Efv4AwCwU Me3/aiCMqic/U+lrFn6b6sl/z1v8HaCaO8TW/ULK57oYcTsJUTT40SC7IWg0qynyolrWCF SwQVdk9jD9CTvuX7/umZfWs44fNsTrQ= ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=NbRDLF5b; spf=pass (imf13.hostedemail.com: domain of frederic@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=frederic@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1761749583; a=rsa-sha256; cv=none; b=fWHG5CwontmyCXLHOUxtGNmF2HY54Yi/aQpQlB7MCBbUH1evf10R8KfCAXfAqVp51hkQDY AlicGMYb8WsVXr7IEJc6a1Rgytrs5v/3AbpPn12ODKVrVymgNEbT/1MtXrpnIk3MsTsiop YzcWMYRjnUXDVod5hPHPsVmkQh4gRNo= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 12E3741728; Wed, 29 Oct 2025 14:53:02 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 39C64C19423; Wed, 29 Oct 2025 14:53:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1761749581; bh=DycaBBkGuqgEIP5677gSgx8QDJGofxkMNPMcP3M730o=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=NbRDLF5bQ+u4J4hiySQAVpBuc86Gn1rrbceqZEKYNWpKF9yoAhVdLiG/Nu9SRzin5 ddwOS2DAvBL0w6K59co5lqXWGrpHXdyA3+e94n4XKlatrRRprHxYDEy/DFtpqcJYjz jGBmveuocLom0Tr99M8s/YfJisMETw3u7WfPV8VPmObP73KGTMrcXJF2ZKhuh5kL4K FNZiQ5jjE+Qy5WcINDDA3sOHLfgNQyF9ljeM8CBlvKWvkNPOMLvSsJkY31UBeqPnRr epdow2onJrow0Coha3Tk5/8X8R1mpOEWDHWRuB0t8h2sykd3/evOgK+UEPDUs1Kui5 FM7cF5Ys9ZgFA== Date: Wed, 29 Oct 2025 15:52:58 +0100 From: Frederic Weisbecker To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Nicolas Saenz Julienne , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Arnaldo Carvalho de Melo , Josh Poimboeuf , Paolo Bonzini , Arnd Bergmann , "Paul E. McKenney" , Jason Baron , Steven Rostedt , Ard Biesheuvel , Sami Tolvanen , "David S. Miller" , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Mel Gorman , Andrew Morton , Masahiro Yamada , Han Shen , Rik van Riel , Jann Horn , Dan Carpenter , Oleg Nesterov , Juri Lelli , Clark Williams , Yair Podemsky , Marcelo Tosatti , Daniel Wagner , Petr Tesarik Subject: Re: [PATCH v6 23/29] context-tracking: Introduce work deferral infrastructure Message-ID: References: <20251010153839.151763-1-vschneid@redhat.com> <20251010153839.151763-24-vschneid@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 44F3020007 X-Stat-Signature: m8twz7up5zmrzq8qoa9q68yboqrxa1ju X-Rspam-User: X-HE-Tag: 1761749583-608519 X-HE-Meta: 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 b0A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Le Wed, Oct 29, 2025 at 11:09:50AM +0100, Valentin Schneider a écrit : > On 28/10/25 15:00, Frederic Weisbecker wrote: > > Le Fri, Oct 10, 2025 at 05:38:33PM +0200, Valentin Schneider a écrit : > >> + old = atomic_read(&ct->state); > >> + > >> + /* > >> + * The work bit must only be set if the target CPU is not executing > >> + * in kernelspace. > >> + * CT_RCU_WATCHING is used as a proxy for that - if the bit is set, we > >> + * know for sure the CPU is executing in the kernel whether that be in > >> + * NMI, IRQ or process context. > >> + * Set CT_RCU_WATCHING here and let the cmpxchg do the check for us; > >> + * the state could change between the atomic_read() and the cmpxchg(). > >> + */ > >> + old |= CT_RCU_WATCHING; > > > > Most of the time, the task should be either idle or in userspace. I'm still not > > sure why you start with a bet that the CPU is in the kernel with RCU watching. > > > > Right I think I got that the wrong way around when I switched to using > CT_RCU_WATCHING vs CT_STATE_KERNEL. That wants to be > > old &= ~CT_RCU_WATCHING; > > i.e. bet the CPU is NOHZ-idle, if it's not the cmpxchg fails and we don't > store the work bit. Right. > > >> + /* > >> + * Try setting the work until either > >> + * - the target CPU has entered kernelspace > >> + * - the work has been set > >> + */ > >> + do { > >> + ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START)); > >> + } while (!ret && !(old & CT_RCU_WATCHING)); > > > > So this applies blindly to idle as well, right? It should work but note that > > idle entry code before RCU watches is also fragile. > > > > Yeah I remember losing some hair trying to grok the idle entry situation; > we could keep this purely NOHZ_FULL and have the deferral condition be: > > (ct->state & CT_STATE_USER) && !(ct->state & CT_RCU_WATCHING) Well, after all what works for NOHZ_FULL should also work for idle. It's preceded by entry code as well (or rather __cpuidle). Thanks. -- Frederic Weisbecker SUSE Labs