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=-12.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 D4211C6369E for ; Thu, 19 Nov 2020 13:13:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7966F246B8 for ; Thu, 19 Nov 2020 13:13:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="0SFglOT5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727663AbgKSNNL (ORCPT ); Thu, 19 Nov 2020 08:13:11 -0500 Received: from mail.kernel.org ([198.145.29.99]:35484 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727449AbgKSNNK (ORCPT ); Thu, 19 Nov 2020 08:13:10 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (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 E254E208D5; Thu, 19 Nov 2020 13:13:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605791589; bh=TJ8b6OEHzQ0ZsijVy8i49yto5LykArD7whycPru1dfQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=0SFglOT5POOg+MPN2NEy+c5vRQY8lV/NX0WhBaJb07p0eXXoTyoebZDl6raTtp8yd rkPh/gQB+K6NtVsWWtNfGiBtYcQkHvRfrvP87yRcvv824VYjReJ6ll540CWUE/EYxG vWUQ5qzmd86rRcfdbV0vlowo9xV00aZMY+Z0rV9I= Date: Thu, 19 Nov 2020 13:13:02 +0000 From: Will Deacon To: Valentin Schneider Cc: Quentin Perret , linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Peter Zijlstra , Morten Rasmussen , Qais Yousef , Suren Baghdasaryan , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , kernel-team@android.com Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity Message-ID: <20201119131301.GD4331@willie-the-truck> References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> <20201119091820.GA2416649@google.com> <20201119110549.GA3946@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). Will 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=-10.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 44D69C63697 for ; Thu, 19 Nov 2020 13:14:33 +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 BFEC0208D5 for ; Thu, 19 Nov 2020 13:14:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="LuMy4fbs"; 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Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfjkP-0005BO-TY; Thu, 19 Nov 2020 13:13:13 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfjkM-00059i-5Z for linux-arm-kernel@lists.infradead.org; Thu, 19 Nov 2020 13:13:11 +0000 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (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 E254E208D5; Thu, 19 Nov 2020 13:13:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605791589; bh=TJ8b6OEHzQ0ZsijVy8i49yto5LykArD7whycPru1dfQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=0SFglOT5POOg+MPN2NEy+c5vRQY8lV/NX0WhBaJb07p0eXXoTyoebZDl6raTtp8yd rkPh/gQB+K6NtVsWWtNfGiBtYcQkHvRfrvP87yRcvv824VYjReJ6ll540CWUE/EYxG vWUQ5qzmd86rRcfdbV0vlowo9xV00aZMY+Z0rV9I= Date: Thu, 19 Nov 2020 13:13:02 +0000 From: Will Deacon To: Valentin Schneider Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity Message-ID: <20201119131301.GD4331@willie-the-truck> References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> <20201119091820.GA2416649@google.com> <20201119110549.GA3946@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201119_081310_377954_C065EC48 X-CRM114-Status: GOOD ( 22.83 ) 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 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). Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel