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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C6D9AC54E76 for ; Tue, 17 Jan 2023 18:50:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235256AbjAQStC (ORCPT ); Tue, 17 Jan 2023 13:49:02 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33308 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231962AbjAQSnT (ORCPT ); Tue, 17 Jan 2023 13:43:19 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EED12568B9 for ; Tue, 17 Jan 2023 10:13:38 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1673979217; 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=HufvuJj0PnCt4I7SANdySEcEJEXB3rZxA1StQp+0Rnc=; b=cVpmDfehKJ4S7H+XxR2BlK69jGSduFIK6bCcm63TkF0OeLtfM8yqYjUvoIL3X+EAB9LlyK U81+OQQ+Eo4pBrr851PkZcCtiICSYuzsnLyQs3hOEUEm8JMj5HLLFa1zkmDoA6BkIicic9 iPic48gxl0wJXprLJYEz1fSYiY63vGQ= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-149-W3XY9xpoMwyQlpnFrT1cIQ-1; Tue, 17 Jan 2023 13:13:33 -0500 X-MC-Unique: W3XY9xpoMwyQlpnFrT1cIQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7BDF529AA3B1; Tue, 17 Jan 2023 18:13:32 +0000 (UTC) Received: from [10.18.17.153] (dhcp-17-153.bos.redhat.com [10.18.17.153]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8246A175A2; Tue, 17 Jan 2023 18:13:31 +0000 (UTC) Message-ID: <2bb8c031-03a9-ef93-1505-6e7fbcc6d847@redhat.com> Date: Tue, 17 Jan 2023 13:13:31 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH v10 2/5] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity() Content-Language: en-US To: Will Deacon Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider , Tejun Heo , Zefan Li , Johannes Weiner , linux-kernel@vger.kernel.org, Linus Torvalds , Lai Jiangshan , qperret@google.com References: <20220922180041.1768141-1-longman@redhat.com> <20220922180041.1768141-3-longman@redhat.com> <20230117160825.GA17756@willie-the-truck> From: Waiman Long In-Reply-To: <20230117160825.GA17756@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/17/23 11:08, Will Deacon wrote: > Hi Waiman, > > On Thu, Sep 22, 2022 at 02:00:38PM -0400, Waiman Long wrote: >> The user_cpus_ptr field is added by commit b90ca8badbd1 ("sched: >> Introduce task_struct::user_cpus_ptr to track requested affinity"). It >> is currently used only by arm64 arch due to possible asymmetric CPU >> setup. This patch extends its usage to save user provided cpumask >> when sched_setaffinity() is called for all arches. With this patch >> applied, user_cpus_ptr, once allocated after a successful call to >> sched_setaffinity(), will only be freed when the task exits. >> >> Since user_cpus_ptr is supposed to be used for "requested >> affinity", there is actually no point to save current cpu affinity in >> restrict_cpus_allowed_ptr() if sched_setaffinity() has never been called. >> Modify the logic to set user_cpus_ptr only in sched_setaffinity() and use >> it in restrict_cpus_allowed_ptr() and relax_compatible_cpus_allowed_ptr() >> if defined but not changing it. >> >> This will be some changes in behavior for arm64 systems with asymmetric >> CPUs in some corner cases. For instance, if sched_setaffinity() >> has never been called and there is a cpuset change before >> relax_compatible_cpus_allowed_ptr() is called, its subsequent call will >> follow what the cpuset allows but not what the previous cpu affinity >> setting allows. >> >> Signed-off-by: Waiman Long >> --- >> kernel/sched/core.c | 82 ++++++++++++++++++++------------------------ >> kernel/sched/sched.h | 7 ++++ >> 2 files changed, 44 insertions(+), 45 deletions(-) > We've tracked this down as the cause of an arm64 regression in Android and I've > reproduced the issue with mainline. > > Basically, if an arm64 system is booted with "allow_mismatched_32bit_el0" on > the command-line, then the arch code will (amongst other things) call > force_compatible_cpus_allowed_ptr() and relax_compatible_cpus_allowed_ptr() > when exec()'ing a 32-bit or a 64-bit task respectively. IOW, relax_compatible_cpus_allowed_ptr() can be called without a previous force_compatible_cpus_allowed_ptr(). Right? A possible optimization in this case is to add a bit flag in the task_struct to indicate a previous call to force_compatible_cpus_allowed_ptr(). Without that flag set, relax_compatible_cpus_allowed_ptr() can return immediately. > > If you consider a system where everything is 64-bit but the cmdline option > above is present, then the call to relax_compatible_cpus_allowed_ptr() isn't > expected to do anything in this case, and the old code made sure of that: > >> @@ -3055,30 +3032,21 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask); >> >> /* >> * Restore the affinity of a task @p which was previously restricted by a >> - * call to force_compatible_cpus_allowed_ptr(). This will clear (and free) >> - * @p->user_cpus_ptr. >> + * call to force_compatible_cpus_allowed_ptr(). >> * >> * It is the caller's responsibility to serialise this with any calls to >> * force_compatible_cpus_allowed_ptr(@p). >> */ >> void relax_compatible_cpus_allowed_ptr(struct task_struct *p) >> { >> - struct cpumask *user_mask = p->user_cpus_ptr; >> - unsigned long flags; >> + int ret; >> >> /* >> - * Try to restore the old affinity mask. If this fails, then >> - * we free the mask explicitly to avoid it being inherited across >> - * a subsequent fork(). >> + * Try to restore the old affinity mask with __sched_setaffinity(). >> + * Cpuset masking will be done there too. >> */ >> - if (!user_mask || !__sched_setaffinity(p, user_mask)) >> - return; > ... since it returned early here if '!user_mask' ... The flag bit will work like the user_mask check here. > >> - >> - raw_spin_lock_irqsave(&p->pi_lock, flags); >> - user_mask = clear_user_cpus_ptr(p); >> - raw_spin_unlock_irqrestore(&p->pi_lock, flags); >> - >> - kfree(user_mask); >> + ret = __sched_setaffinity(p, task_user_cpus(p)); >> + WARN_ON_ONCE(ret); > ... however, now we end up going down into __sched_setaffinity() with > task_user_cpus(p) giving us the 'cpu_possible_mask'! This can lead to a mixture > of WARN_ON()s and incorrect affinity masks (for example, a newly exec'd task > ends up with the affinity mask of the online CPUs at the point of exec() and is > unable to run on anything onlined later). CPU hotplug should update the cpumask of existing running application as allowed by cpuset. > > I've had a crack at fixing the code above to restore the old behaviour, and it > seems to work for my basic tests (still pending confirmation from others): > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index bb1ee6d7bdde..0d4a11384648 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3125,17 +3125,16 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx); > void relax_compatible_cpus_allowed_ptr(struct task_struct *p) > { > struct affinity_context ac = { > - .new_mask = task_user_cpus(p), > + .new_mask = p->user_cpus_ptr, > .flags = 0, > }; > - int ret; > > /* > * Try to restore the old affinity mask with __sched_setaffinity(). > * Cpuset masking will be done there too. > */ > - ret = __sched_setaffinity(p, &ac); > - WARN_ON_ONCE(ret); > + if (ac.new_mask) > + WARN_ON_ONCE(__sched_setaffinity(p, &ac)); > } > > void set_task_cpu(struct task_struct *p, unsigned int new_cpu) > > > With this change, task_user_cpus() is only used by restrict_cpus_allowed_ptr() > so I'd be inclined to remove it altogether tbh. > > What do you think? The problem here is that force_compatible_cpus_allowed_ptr() can be called without a matching relax_compatible_cpus_allowed_ptr() at the end. So we may end up artificially restrict the number of cpus that can be used when running a 64-bit binary. What do you think about the idea of having a bit flag to track that? Cheers, Longman