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_PATCH, 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 18D56C6369E for ; Thu, 19 Nov 2020 14:55:22 +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 8E09122261 for ; Thu, 19 Nov 2020 14:55:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="pCB18cN8" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8E09122261 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=jccBL22OMopfpDeObymBtYSujjJ94e+0oAXLNGM/D+o=; b=pCB18cN8RhtYyVjsXZXiJ0o9E 5Qa+YbMAo0uXtH4eK94veGa2Y6nimRRrb3k73mvKhOiGKp04lIyqkSxLJm0/8LxDA6EP1cTHYwE8X 2wMoQfqir+CuO3+D0gn1n2L/emkfxQ4KIVgZJaFe7+yWv6O9XP9b+ytHqzhUBd8o1frrRufuDvdkZ KF4hC6zcjxs7NP4FsHKJrJaen0JSwMSzv687uVF8tVt8/GDoL7LTh6WK4qMCPIsPLQA5bPDv3Q9b8 F9ldR53L8mr1tk626sONAxKj/MN9ZNI0UK12LS1Fiz1iAz8zRHfEX4I3ZLnGoFjbm/oRY9n8nSuCM kxz4/zvbA==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kflKf-0006tD-Sq; Thu, 19 Nov 2020 14:54:45 +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 1kflKb-0006rz-OV for linux-arm-kernel@lists.infradead.org; Thu, 19 Nov 2020 14:54:43 +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 CDDAD1042; Thu, 19 Nov 2020 06:54:40 -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 7FB233F719; Thu, 19 Nov 2020 06:54:38 -0800 (PST) References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> <20201119091820.GA2416649@google.com> <20201119110549.GA3946@willie-the-truck> <20201119131301.GD4331@willie-the-truck> 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: <20201119131301.GD4331@willie-the-truck> Date: Thu, 19 Nov 2020 14:54:32 +0000 Message-ID: MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201119_095441_905633_F18539E3 X-CRM114-Status: GOOD ( 20.27 ) 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, Juri Lelli , kernel-team@android.com, Vincent Guittot , Quentin Perret , Peter Zijlstra , Catalin Marinas , Johannes Weiner , linux-kernel@vger.kernel.org, Qais Yousef , Ingo Molnar , Li Zefan , Greg Kroah-Hartman , Marc Zyngier , 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 19/11/20 13:13, Will Deacon wrote: > On Thu, Nov 19, 2020 at 11:27:55AM +0000, Valentin Schneider wrote: >> >> On 19/11/20 11:05, Will Deacon wrote: >> > On Thu, Nov 19, 2020 at 09:18:20AM +0000, Quentin Perret wrote: >> >> > @@ -1937,20 +1931,69 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, >> >> > * OK, since we're going to drop the lock immediately >> >> > * afterwards anyway. >> >> > */ >> >> > - rq = move_queued_task(rq, &rf, p, dest_cpu); >> >> > + rq = move_queued_task(rq, rf, p, dest_cpu); >> >> > } >> >> > out: >> >> > - task_rq_unlock(rq, p, &rf); >> >> > + task_rq_unlock(rq, p, rf); >> >> >> >> And that's a little odd to have here no? Can we move it back on the >> >> caller's side? >> > >> > I don't think so, unfortunately. __set_cpus_allowed_ptr_locked() can trigger >> > migration, so it can drop the rq lock as part of that and end up relocking a >> > new rq, which it also unlocks before returning. Doing the unlock in the >> > caller is therfore even weirder, because you'd have to return the lock >> > pointer or something horrible like that. >> > >> > I did add a comment about this right before the function and it's an >> > internal function to the scheduler so I think it's ok. >> > >> >> An alternative here would be to add a new SCA_RESTRICT flag for >> __set_cpus_allowed_ptr() (see migrate_disable() faff in >> tip/sched/core). Not fond of either approaches, but the flag thing would >> avoid this "quirk". > > I tried this when I read about the migrate_disable() stuff on lwn, but I > didn't really find it any better to work with tbh. It also doesn't help > with the locking that Quentin was mentioning, does it? (i.e. you still > have to allocate). > You could keep it all bundled within __set_cpus_allowed_ptr() (i.e. not have a _locked() version) and use the flag as indicator of any extra work. Also FWIW we have this pattern of pre-allocating pcpu cpumasks (select_idle_mask, load_balance_mask), but given this is AIUI a very-not-hot path, this might be overkill (and reusing an existing one would be on the icky side of things). > Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel