From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 CEA4A47F3DE for ; Thu, 15 Oct 2026 20:14:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792095244; cv=none; b=R1SPGKFBhlGUHjHb6TwChd4MKHJU+bkvm6E/ZzpGvVzcrc2A/SWopOjxYuT+Krwc0zqRnjl5f3H6oq2v2DkcYQx9+xlx+GXgq62/8qz+OBoXEZlRuglspFGa69SXF0+ObstSEtK7I/6gntRU7pOrh3AQyeG4sjj2dwXoKzWCiXw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792095244; c=relaxed/simple; bh=SnbfDLY4tTJqB1v8LhA/WdScgHkhh06wd2LjFSIubrE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hKEZ2cjdnVpGp5H83yFSP138Cs94AM4DuPglz6xM5W3OXtAQ7cX2j50wz1DSyj7zTXhiZSckF43MvG9U8Yw4ykpKfP4G6ws4p8Wv5SEGAvYYK8Xt6ew9IgUlYQoi7/paxo9ot2kekFWWrG2F7GPwmC0QCMckJIArqgxu1f7iSYg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=heqTmse4; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="heqTmse4" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1792095241; h=from:from: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; bh=UYRSFyiULvh6szKRePAOtHKD+7/o0oF90IjOubYIGB4=; b=heqTmse4IcYcxajpUHFO5NfmHq45vspk7SJEh7yI5uWAQF/Q7MV1MGN/4EYI4t5lyQXOAw 8hLacwH5qhmwIBBpy2po31mZgoSB366sQomyd0JdUYRle6wgmhoL3Vaa8QFdBCZxIdfgaT RQWUpHh1uoo3+F2GpJDj/xhcPYM7k5k= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-600-IdpI9AU0N62UZVqT0kj7fQ-1; Thu, 15 Oct 2026 20:13:56 +0000 X-MC-Unique: IdpI9AU0N62UZVqT0kj7fQ-1 X-Mimecast-MFC-AGG-ID: IdpI9AU0N62UZVqT0kj7fQ_1792095234 Received: from mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.95]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A9BBB1955F1A; Thu, 15 Oct 2026 20:13:53 +0000 (UTC) Received: from [100.91.18.181] (headnet03.pony-001.prod.iad2.dc.redhat.com [10.2.32.114]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 6D8397C6; Thu, 15 Oct 2026 20:13:50 +0000 (UTC) Message-ID: Date: Thu, 15 Oct 2026 16:13:49 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/2] tick/nohz: Add runtime control of non-idle tick suppression To: Peng Wang Cc: Anna-Maria Behnsen , Frederic Weisbecker , Ingo Molnar , Thomas Gleixner , Greg Kroah-Hartman , "Rafael J . Wysocki" , Danilo Krummrich , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Peter Zijlstra , Tejun Heo , He Rongguang , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, driver-core@lists.linux.dev, linux-trace-kernel@vger.kernel.org, cgroups@vger.kernel.org References: <8d10bb84-d312-4be0-8c43-ddd602850a1d@redhat.com> Content-Language: en-US From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 On 10/15/26 1:48 PM, Peng Wang wrote: > On Thu, Oct 15, 2026 at 12:00:25PM -0400, Waiman Long wrote: >> On 10/15/26 8:43 AM, Peng Wang wrote: >>> Workloads and their need for periodic ticks can change after boot. >>> This series lets userspace choose at runtime which CPUs in the boot-time >>> nohz_full set retain the non-idle scheduler tick and which are permitted >>> to stop it, while preserving the existing isolation configuration. >>> >>> The first patch adds a per-CPU tick dependency; the second exposes a >>> sysfs CPU list. Permission defaults to all nohz_full CPUs. Revocation >>> uses the existing asynchronous kick. Idle tick suppression is unchanged, >>> and policy persists across hotplug. >> So you are basically setting up more nohz_full CPUs than the system actually >> needs to use at the moment and so want to disable nohz/tick part of >> nohz_full CPUs that don't need it. Can you elaborate more about the problems >> solve by introducing this new behavior? > Hi Waiman, > > Yes. We have CPU-pinned workloads that benefit from reduced periodic > tick interference. The desired tick policy changes with CPU allocation. > > With spare capacity, we allow selected nohz_full CPUs to stop their > tick. When capacity is fully allocated, we prefer to retain the > non-idle periodic tick across CPUs, avoiding an additional difference > in tick interference between housekeeping and nohz_full CPUs. What problem does that cause? > A CPU may still have only one runnable task in either case, so the > existing tick dependencies do not express this policy. > > This series keeps the boot-time nohz_full CPU set, context tracking, > RCU offloading and housekeeping configuration unchanged. It only > controls whether those CPUs may stop their non-idle tick. The policy > can be changed without taking CPUs offline. So the policy is to make the tick keeps running when there is only one running task. Right? I am just not clear about the advantage of doing that. Is there a use case that will require this particular feature? I doubt this will get accepted unless there is a compelling reason to show why we will want to have this feature. Cheers, Longman