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=-8.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 A4190C433E7 for ; Fri, 9 Oct 2020 08:51:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 54E6322275 for ; Fri, 9 Oct 2020 08:51:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732712AbgJIIvC (ORCPT ); Fri, 9 Oct 2020 04:51:02 -0400 Received: from foss.arm.com ([217.140.110.172]:45126 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730726AbgJIIvB (ORCPT ); Fri, 9 Oct 2020 04:51:01 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3E707D6E; Fri, 9 Oct 2020 01:51:01 -0700 (PDT) Received: from e123083-lin (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DE0393F66B; Fri, 9 Oct 2020 01:50:59 -0700 (PDT) Date: Fri, 9 Oct 2020 10:50:57 +0200 From: Morten Rasmussen To: Will Deacon Cc: linux-arch@vger.kernel.org, Marc Zyngier , Peter Zijlstra , Greg Kroah-Hartman , Catalin Marinas , Linus Torvalds , Qais Yousef , linux-arm-kernel@lists.infradead.org Subject: Re: [RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu Message-ID: <20201009085057.GB8004@e123083-lin> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201009083146.GA29594@willie-the-truck> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org 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. I agree it doesn't make sense to hide the asymmetry. The only argument I see for tweaking the affinity is to be more friendly in case user-space is unaware. > 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 I'm fine with that. We just have to accept that existing unaware user-space(s) may see tasks getting killed if they use task affinity. Morten