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 X-Spam-Level: X-Spam-Status: No, score=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1EB20C433DF for ; Fri, 9 Oct 2020 11:39:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8FB0F22261 for ; Fri, 9 Oct 2020 11:39:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="bWHt8+GU" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388138AbgJILjU (ORCPT ); Fri, 9 Oct 2020 07:39:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55200 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731908AbgJILjU (ORCPT ); Fri, 9 Oct 2020 07:39:20 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E862FC0613D2 for ; Fri, 9 Oct 2020 04:39:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=jUWvupSS4x5Jc67h2CeSW+ULhwAq8rOUFqfMfL42U8Q=; b=bWHt8+GUjByoxNEQeA57hs9di7 i7mGxYz3T/cym89tzhgnlcxiqTF2dGMHenx1Qmheg1uM4Sb5Pdg3cbsKad6YFKaBdXeXCRZenS1io Ec6LVYwWdaOnzTporuIxjsBJnED7gXmbse0kCYVTJiGJ0Nw91zd522X/GShkPeFGSO/ICAv+l4gsw 9m21Q+jkq57NF/cNmI/m5IH7nHK3gAsChQJ9JE6SImzKa/w4Ke1CWOFzmfyi4ebHavd1fvklLkjad DGZBNGPL1ujc8ywEzBYsq9Yhh5Y9nLEO4Wtv5lJfmptQtYjTptJM0kWQvCgd11fHUVsSyAin6VuLh 7ENGwByg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kQqjy-0003hu-03; Fri, 09 Oct 2020 11:39:14 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 68F6530008D; Fri, 9 Oct 2020 13:39:09 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 4E2A329AE66EF; Fri, 9 Oct 2020 13:39:09 +0200 (CEST) Date: Fri, 9 Oct 2020 13:39:09 +0200 From: Peter Zijlstra To: Michal Hocko Cc: Thomas Gleixner , Frederic Weisbecker , LKML , Mel Gorman , Ingo Molnar Subject: Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE, PREEMPT_VOLUNTARY on kernel command line Message-ID: <20201009113909.GL2628@hirez.programming.kicks-ass.net> References: <20201007120401.11200-1-mhocko@kernel.org> <20201007122144.GF2628@hirez.programming.kicks-ass.net> <20201007123553.GK29020@dhcp22.suse.cz> <20201009094741.GH2628@hirez.programming.kicks-ass.net> <20201009101405.GI4967@dhcp22.suse.cz> <20201009102009.GK2628@hirez.programming.kicks-ass.net> <20201009104808.GK4967@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201009104808.GK4967@dhcp22.suse.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 09, 2020 at 12:48:08PM +0200, Michal Hocko wrote: > On Fri 09-10-20 12:20:09, Peter Zijlstra wrote: > > On Fri, Oct 09, 2020 at 12:14:05PM +0200, Michal Hocko wrote: > > > On Fri 09-10-20 11:47:41, Peter Zijlstra wrote: > > > > > > That is, work backwards (from PREEMPT back to VOLUNTARY) instead of the > > > > other way around. > > > > > > My original idea was that the config would only define the default > > > preemption mode. preempt_none parameter would then just act as an > > > override. That would mean that CONFIG_PREEMPTION would be effectively > > > gone from the kernel. The reason being that any code outside of the > > > scheduler shouldn't really care about the preemption mode. I suspect > > > this will prevent from dubious hacks and provide a more robust code in > > > the end. > > > > Sure; but the way of arriving at that destination might be easier if > > you work backwards from PREEMPT=y, because while there _should_ not be > > dependencies outside of the scheduler, we both know there are. > > Wouldn't we need to examine each of the CONFIG_PREEMPTION code anyway? > And wouldn't that be even more tricky? The boot time option would result > in a more restrictive preemption mode while the code is actually > assuming a less restrictive one. Sure, in the end we'll have to look at all of that. > > This also makes your patches independent of the series that makes > > CONFIG_PREEMPTION unconditional. > > > > It also gives Kconfig space to limit the dynamic thing to archs that > > have sufficient support (we'll be relying on static_call/static_branch, > > and not everybody has that implemented in a way that makes it the > > dynamic change worth-while). > > Hmm, this is actually a good argument. I can imagine that kernels > without CONFIG_JUMP_LABEL might increase a runtime overhead for > something that users of that kernel might be not really interested in. > This would make CONFIG_PREEMPT_DYNAMIC be selected by CONFIG_JUMP_LABEL. > > I will add the CONFIG_PREEMPT_DYNAMIC in the next version. I just have > to think whether flipping the direction is really safe and easier in the > end. For our particular usecase we are more interested in > NONE<->VOLUNTARY at this moment and having full preemption in the mix > later is just fine. If you insist on the other direction then we can > work on that. Ah, I was purely thinking of the FULL preempt case. For the NONE/VOLATILE case you can probably keep it simpler.