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=-8.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 15B3EC63697 for ; Thu, 19 Nov 2020 12:48:18 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 995C820771 for ; Thu, 19 Nov 2020 12:48:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="m8+zJi66" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 995C820771 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:MIME-Version:Message-ID:Date:In-reply-to:Subject:To: From:References:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=o0RgGwsO53TjXWFYdFxny+dwKZEOAUU19PgdIUC79Co=; b=m8+zJi66HtVDBPx9OjXTa9V7I g+x4BFTDkkiIFsKf+2JwficH5SJm30mJUfdM9iE+8xx7jryn8zc1mhLr2n+R5Lfr5GG7K96H7KOyL Zg/h4nZOSJvqnt9e4Ja+G4pb7/icR6SNquEpAU70AoASKcRFmMUdSQ3Doei1jXbFnAvKvAKbgDOJN d5FjNB4dfkNLUzWlXSqVdvK1XqZFojafi4n/9blKNePCEKfq6FV4TZoT77ANKiZ9l2UR5RKovuzdm Q50BiHIZRWNypnaLeXlnoi37eCj1rMqxhecKVb2/UvE07QCDJaNYYsP4je7T/mjoax6O7VayCSV+O KDyXLu0cA==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfjLm-0000lN-5U; Thu, 19 Nov 2020 12:47:46 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfjLh-0000jx-MO for linux-arm-kernel@lists.infradead.org; Thu, 19 Nov 2020 12:47:42 +0000 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 D7FD21396; Thu, 19 Nov 2020 04:47:38 -0800 (PST) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8C0093F718; Thu, 19 Nov 2020 04:47:36 -0800 (PST) References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Will Deacon Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity In-reply-to: <20201113093720.21106-8-will@kernel.org> Date: Thu, 19 Nov 2020 12:47:34 +0000 Message-ID: MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201119_074741_831347_11F810E5 X-CRM114-Status: GOOD ( 17.00 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Marc Zyngier , kernel-team@android.com, Vincent Guittot , Juri Lelli , Quentin Perret , Peter Zijlstra , Catalin Marinas , Johannes Weiner , linux-kernel@vger.kernel.org, Qais Yousef , Ingo Molnar , Li Zefan , Greg Kroah-Hartman , Tejun Heo , Suren Baghdasaryan , Morten Rasmussen , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 13/11/20 09:37, Will Deacon wrote: > Asymmetric systems may not offer the same level of userspace ISA support > across all CPUs, meaning that some applications cannot be executed by > some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do > not feature support for 32-bit applications on both clusters. > > Although userspace can carefully manage the affinity masks for such > tasks, one place where it is particularly problematic is execve() > because the CPU on which the execve() is occurring may be incompatible > with the new application image. In such a situation, it is desirable to > restrict the affinity mask of the task and ensure that the new image is > entered on a compatible CPU. > From userspace's point of view, this looks the same as if the > incompatible CPUs have been hotplugged off in its affinity mask. {pedantic reading warning} Hotplugged CPUs *can* be set in a task's affinity mask, though interact weirdly with cpusets [1]. Having it be the same as hotplug would mean keeping incompatible CPUs allowed in the affinity mask, but preventing them from being picked via e.g. is_cpu_allowed(). I was actually hoping this could be a feasible approach, i.e. have an extra CPU active mask filter for any task: cpu_active_mask & arch_cpu_allowed_mask(p) rather than fiddle with task affinity. Sadly this would also require fixing up pretty much any place that uses cpu_active_mask(), and probably places that use p->cpus_ptr as well. RT / DL balancing comes to mind, because that uses either a task's affinity or a CPU's root domain (which reflects the cpu_active_mask) to figure out where to push a task. So while I'm wary of hacking up affinity, I fear it might be the lesser evil :( [1]: https://lore.kernel.org/lkml/1251528473.590671.1579196495905.JavaMail.zimbra@efficios.com/ > In preparation for restricting the affinity mask for compat tasks on > arm64 systems without uniform support for 32-bit applications, introduce > a restrict_cpus_allowed_ptr(), which allows the current affinity mask > for a task to be shrunk to the intersection of a parameter mask. > > Signed-off-by: Will Deacon _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel