From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 6E24F1DED5B; Fri, 24 Apr 2026 15:57:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777046267; cv=none; b=fHlddOl9rDzcTI3iweNdh2/A0a3gcyDUwndX0+yYkehoFwPlG394VWDZsxJ4oY79w9JUsx8zRpsJJfsqCF7KcTNfaaXfxByFk2H6jI8GFw5RPl5mswihr2Ei83UqmHgDOJWkKjF1QwNrU7wTw/cnLiIWfscyjU8t+9I3xc79edU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777046267; c=relaxed/simple; bh=qO/MAUTAHfRFlyhPD2rItuN3QmG4P8rtxuTLbkxNwL0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JmsT/jq26HBpxr2QofPWP7j6OCG74VoZYZYRz0K8pR7nNuSaHcDzA8ABv1ZVHOTwZO/ncQpwAIY0Ff1mUGMSWhXRFfVjSKN7Ihq+S0sZZAc4JslppF/gpgS3FR+4jIbnoJUYJY8lXWFUoS50mYdlf1xk0YCqPyMPcu308pk15dA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ajfozHwD; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ajfozHwD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 84C95C2BCB6; Fri, 24 Apr 2026 15:57:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777046267; bh=qO/MAUTAHfRFlyhPD2rItuN3QmG4P8rtxuTLbkxNwL0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ajfozHwD1fCyxVIfcezb1FBnsxwPft4s+SO1HNKvMY5sayIllGVwXOVtVSslBiMZF iBATPF8Gcw9+n7ropEBl/64VF34ptpNInOVsEsJkrETPnxJJKygFlN1VYMgsTk/aYJ gYUvbgp7JalTMlmRCPOXSELTijM0xY26ZJKLVNhvwHiNjYbLxtQV/g5iAlxMKVWK29 RGgC4BQzNUj5QUpG0M5muBCkcgoXJy+cqEnzLKsOM+NOcQcatVhMlBI26zUciOzkDE k9Zm/lBjzIrlUV4Mjw8X3a4NKdoQ0/dEMltXuRb3fUxMSZ+xWpW5L0buiDsXXrS+bR AlN0XSo5XZv8Q== Date: Fri, 24 Apr 2026 17:57:43 +0200 From: Frederic Weisbecker To: Waiman Long Cc: Thomas Gleixner , Tejun Heo , Johannes Weiner , Michal =?iso-8859-1?Q?Koutn=FD?= , Jonathan Corbet , Shuah Khan , Catalin Marinas , Will Deacon , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Long Li , Guenter Roeck , "Paul E. McKenney" , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Anna-Maria Behnsen , Ingo Molnar , Chen Ridong , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-hyperv@vger.kernel.org, linux-hwmon@vger.kernel.org, rcu@vger.kernel.org, netdev@vger.kernel.org, linux-kselftest@vger.kernel.org, Costa Shulyupin , Qiliang Yuan Subject: Re: [PATCH 03/23] tick/nohz: Make nohz_full parameter optional Message-ID: References: <20260421030351.281436-1-longman@redhat.com> <20260421030351.281436-4-longman@redhat.com> <875x5kd88d.ffs@tglx> <3b796360-81e4-4f90-9b19-8a9f21cbac07@redhat.com> Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3b796360-81e4-4f90-9b19-8a9f21cbac07@redhat.com> Le Tue, Apr 21, 2026 at 10:14:09AM -0400, Waiman Long a écrit : 11;rgb:2e2e/3434/3636> On 4/21/26 4:32 AM, Thomas Gleixner wrote: > > On Mon, Apr 20 2026 at 23:03, Waiman Long wrote: > > > To provide nohz_full tick support, there is a set of tick dependency > > > masks that need to be evaluated on every IRQ and context switch. > > s/IRQ/interrupt/ > > > > This is a changelog and not a SMS service. > > > Switching on nohz_full tick support at runtime will be problematic > > > as some of the tick dependency masks may not be properly set causing > > > problem down the road. > > That's useless blurb with zero content. > > > > > Allow nohz_full boot option to be specified without any > > > parameter to force enable nohz_full tick support without any > > > CPU in the tick_nohz_full_mask yet. The context_tracking_key and > > > tick_nohz_full_running flag will be enabled in this case to make > > > tick_nohz_full_enabled() return true. > > I kinda can crystal-ball what you are trying to say here, but that does > > not make it qualified as a proper change log. > > > > > There is still a small performance overhead by force enable nohz_full > > > this way. So it should only be used if there is a chance that some > > > CPUs may become isolated later via the cpuset isolated partition > > > functionality and better CPU isolation closed to nohz_full is desired. > > Why has this key to be enabled on boot if there are no CPUs in the > > isolated mask? > > > > If you want to manage this dynamically at runtime then enable the key > > once CPUs are isolated. Yes, it's more work, but that avoids the "should > > only be used" nonsense and makes this more robust down the road. > > OK, I will try to make it fully dynamic. Of course, it will be more work. Since the target CPUs will be offline, it should be fine to just enable/disable the static key and masks on runtime. The only issue I see right now is posix CPU timers because the tick dependency is per task/process group. And those tasks could migrate to nohz_full CPUs by careless users (even though that's nonsense) once the target become online. So the per task/process tick dependency must be set up unconditionally. I don't think this should bring much noticeable overhead though. Oh and the other way to go, that is removing TICK_DEP_BIT_POSIX_TIMER and forbid to run posix cpu timers on nohz_full CPUs, would be even more painful to implement so I don't have a better idea. Thanks. -- Frederic Weisbecker SUSE Labs