From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 DE01E395261 for ; Wed, 25 Feb 2026 11:36:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772019425; cv=none; b=FTsM9vEBmdTwDrgvyfGXPbHUjDrIgb2Q94+UGxPW/iCpwFWt7i+VJSFsUDOgNDdE3GEP5Pj3PE8G2FlwvMWUVExMjGY0uHsQzyhhP8iyD4JfkzIZhchfTlqmRRhpgYXRT2WucN3Nw8sVS61hYT0/mtGyofpdTVjREuxJVybN2vE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772019425; c=relaxed/simple; bh=edOh0Huewd6TLZnJxVGH80WBuBYyCGoZ1Mv4ynScbJ0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=brceUixkbhunFbcAteyIyP7avZvxET6RRcSUopXNFZTifd5f6DEac4ZRPhKk1/8xgOBqrcnlIYAwGC9B0MzNi60KpoAQrE3xXxHINzhIepat9aQ9OWR30DE9rG6+/39Rk1ghGsWgtyyqBHoU0NaGKpliAS41PQCICtt1qWfee9g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=YYutkN2I; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="YYutkN2I" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=SW71WeRbACuk909swxb2Z8jt9/wXsC+QTFSAi0G1Dq0=; b=YYutkN2IT/62sokZoK2c9Y2kqm aWfdP9DRYQN6LFXXLAEhiNn40eiz9nh27ym7Nc30EmCCkg0Hokev8d6dPFMS2jRH95Ho3wkFaZ+Nv eth02CdFQRcFOaGBz4IDiWI4PYzfAR3gG/3oB8y74gJIntkvuZZfbea9qYqYdBtqnPA9KjhEzyPs/ ltgpckD47DrT4eqYjv13HFehyrWkR5XLr/1/hUqfrW+XJSyydu4p1cgxsEvJZ9HfmdO59W3/PwFxA +JSIBOlpjqn7mhVMNICNJoIj1JZgaSdoDCtv0aD2BtheYVDZD5BFmY9lmAnoc02xhyHhxJo8ao8NP CIb+Og1Q==; Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vvDCI-00000008u6b-3rjR; Wed, 25 Feb 2026 11:36:55 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 0E2F730331B; Wed, 25 Feb 2026 12:36:53 +0100 (CET) Date: Wed, 25 Feb 2026 12:36:53 +0100 From: Peter Zijlstra To: "Christophe Leroy (CS GROUP)" Cc: Christian Zigotzky , Shrikanth Hegde , linuxppc-dev , "kvm-ppc@vger.kernel.org" , Madhavan Srinivasan , "R.T.Dickinson" , hypexed@yahoo.com.au, mad skateman , Christian Zigotzky Subject: Re: [Linux PPC] Disable PREEMPT Message-ID: <20260225113653.GM3016024@noisy.programming.kicks-ass.net> References: <185d7a42-f7b4-44e1-a129-f5989d211c74@linux.ibm.com> <40154f6a-39da-959d-e039-5ab9ff62db3c@xenosoft.de> <709b864f-9daa-4a48-b3d6-39baedce63b2@linux.ibm.com> <843b7c60-23f1-47d0-b4e8-2147c3e40c23@linux.ibm.com> <2a27e6fc-7eb4-41a5-ba88-58f6a22625b5@xenosoft.de> <1d79ec6f-ce7f-4432-80e0-a54fa1668642@linux.ibm.com> <1c9b879a-7907-4849-a416-5df3db5617d6@kernel.org> Precedence: bulk X-Mailing-List: kvm-ppc@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: <1c9b879a-7907-4849-a416-5df3db5617d6@kernel.org> On Wed, Feb 25, 2026 at 08:16:46AM +0100, Christophe Leroy (CS GROUP) wrote: > +Peter > > Hi Christian, > > Le 25/02/2026 à 06:54, Christian Zigotzky a écrit : > > Hello, > > > > We were previously able to disable PREEMPT in the kernel configuration, > > but the latest kernels now enable it by default and it is no longer > > possible to disable it. > > > > Is it possible to let us decide whether to activate PREEMPT or not? > > Can you give more details on why you want to be able to deactivate PREEMPT ? > > Read https://github.com/torvalds/linux/commit/7dadeaa6e851e Right. So to recap, there are and will be more cases where relying on preemption is mandatory. The typical example is huge page memset/memcpy. Currently that is done in single page chunks with cond_resched() in between, but it is much better (and faster) to do a full huge page in one go. Combine this with the endless stream of mostly random cond_resched() placements due to $reason, and it is all a bit of a maintenance nightmare. I suppose the question is; why do you think you require non-preempt kernels?