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=-6.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, 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 57563C433E7 for ; Fri, 9 Oct 2020 09:41:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED18A2225D for ; Fri, 9 Oct 2020 09:41:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602236489; bh=5rXSXrF9UMJlLLSYZ9O8VLXGTKQzICboyHathAfWgWQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=db/0gtdupIg5OOuM1z+XUoUgjVjlmrHksQAYmnnearU6YR0JtwmP6qUB3Nc8BFodR pe/jSATmTwVdvXJ+EOnmqRte9IdyT85TsdPrCHj9q6JlLqs9IMb0GcyEW3+caOtfyy 0Vzjc7TZ9svoM0sjqpVnntJ62WmWFqo76d1U8LtI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726236AbgJIJl2 (ORCPT ); Fri, 9 Oct 2020 05:41:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:60852 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726211AbgJIJl2 (ORCPT ); Fri, 9 Oct 2020 05:41:28 -0400 Received: from localhost (83-86-74-64.cable.dynamic.v4.ziggo.nl [83.86.74.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C4E702225A; Fri, 9 Oct 2020 09:41:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602236487; bh=5rXSXrF9UMJlLLSYZ9O8VLXGTKQzICboyHathAfWgWQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=uW+tcyvx5JetIV8Gb9OzRxrSh5JbUYQIwDZAKut+Ey7+Cxn4rnCAldKQbM4gUrPdL 058jhu3D2SVYNDot+fGfi+uIf2G3p1tUkrqrOkeEZ8A8mgkcD8dT7cmHG8tfz+552j YmfK7KLHyhws1gOT3ZVuc8cIAZm9sw0/k+9jMjxs= Date: Fri, 9 Oct 2020 11:42:13 +0200 From: Greg Kroah-Hartman To: Catalin Marinas Cc: Will Deacon , Morten Rasmussen , Peter Zijlstra , Qais Yousef , linux-arch@vger.kernel.org, Marc Zyngier , Linus Torvalds , linux-arm-kernel@lists.infradead.org Subject: Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu Message-ID: <20201009094213.GB486675@kroah.com> References: <20201008181641.32767-1-qais.yousef@arm.com> <20201008181641.32767-4-qais.yousef@arm.com> <20201009072943.GD2628@hirez.programming.kicks-ass.net> <20201009081312.GA8004@e123083-lin> <20201009083146.GA29594@willie-the-truck> <20201009093340.GC23638@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201009093340.GC23638@gaia> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Fri, Oct 09, 2020 at 10:33:41AM +0100, Catalin Marinas wrote: > On Fri, Oct 09, 2020 at 09:31:47AM +0100, Will Deacon wrote: > > On Fri, Oct 09, 2020 at 10:13:12AM +0200, Morten Rasmussen wrote: > > > On Fri, Oct 09, 2020 at 09:29:43AM +0200, Peter Zijlstra wrote: > > > > On Thu, Oct 08, 2020 at 07:16:41PM +0100, Qais Yousef wrote: > > > > > diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c > > > > > index cf94cc248fbe..7e97f1589f33 100644 > > > > > --- a/arch/arm64/kernel/signal.c > > > > > +++ b/arch/arm64/kernel/signal.c > > > > > @@ -908,13 +908,28 @@ static void do_signal(struct pt_regs *regs) > > > > > restore_saved_sigmask(); > > > > > } > > > > > > > > > > +static void set_32bit_cpus_allowed(void) > > > > > { > > > > > + cpumask_var_t cpus_allowed; > > > > > + int ret = 0; > > > > > + > > > > > + if (cpumask_subset(current->cpus_ptr, &aarch32_el0_mask)) > > > > > + return; > > > > > + > > > > > /* > > > > > + * On asym aarch32 systems, if the task has invalid cpus in its mask, > > > > > + * we try to fix it by removing the invalid ones. > > > > > */ > > > > > + if (!alloc_cpumask_var(&cpus_allowed, GFP_ATOMIC)) { > > > > > + ret = -ENOMEM; > > > > > + } else { > > > > > + cpumask_and(cpus_allowed, current->cpus_ptr, &aarch32_el0_mask); > > > > > + ret = set_cpus_allowed_ptr(current, cpus_allowed); > > > > > + free_cpumask_var(cpus_allowed); > > > > > + } > > > > > + > > > > > + if (ret) { > > > > > + pr_warn_once("Failed to fixup affinity of running 32-bit task\n"); > > > > > force_sig(SIGKILL); > > > > > } > > > > > } > > > > > > > > Yeah, no. Not going to happen. > > > > > > > > Fundamentally, you're not supposed to change the userspace provided > > > > affinity mask. If we want to do something like this, we'll have to teach > > > > the scheduler about this second mask such that it can compute an > > > > effective mask as the intersection between the 'feature' and user mask. > > > > > > I agree that we shouldn't mess wit the user-space mask directly. Would it > > > be unthinkable to go down the route of maintaining a new mask which is > > > the intersection of the feature mask (controlled and updated by arch > > > code) and the user-space mask? > > > > > > It shouldn't add overhead in the scheduler as it would use the > > > intersection mask instead of the user-space mask, the main complexity > > > would be around making sure the intersection mask is updated correctly > > > (cpusets, hotplug, ...). > > > > > > Like the above tweak, this won't help if the intersection mask is empty, > > > task will still get killed but it will allow tasks to survive > > > user-space masks including some non-compatible CPUs. If we want to > > > prevent task killing in all cases (ignoring hotplug) it gets more ugly > > > as we would have to ignore the user-space mask in some cases. > > > > Honestly, I don't understand why we're trying to hide this asymmetry from > > userspace by playing games with affinity masks in the kernel. Userspace > > is likely to want to move things about _anyway_ because even amongst the > > 32-bit capable cores, you may well have different clock frequencies to > > contend with. > > > > So I'd be *much* happier to let the schesduler do its thing, and if one > > of these 32-bit tasks ends up on a core that can't deal with it, then > > tough, it gets killed. Give userspace the information it needs to avoid > > that happening in the first place, rather than implicitly limit the mask. > > > > That way, the kernel support really boils down to two parts: > > > > 1. Remove the sanity checks we have to prevent 32-bit applications running > > on asymmetric systems > > > > 2. Tell userspace about the problem > > This works for me as well as long as it is default off with a knob to > turn it on. I'd prefer a sysctl (which can be driven from the command > line in recent kernels IIRC) so that one can play with it a run-time. > This way it's also a userspace choice and not an admin or whoever > controls the cmdline (well, that's rather theoretical since the target > is Android). The target _today_ is Android. Chips and cores have a life of their own and end up getting used all over the place over time, as you know :) thanks, greg k-h