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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 6AC25ECDE5F for ; Mon, 23 Jul 2018 14:31:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2276420779 for ; Mon, 23 Jul 2018 14:31:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2276420779 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388468AbeGWPcs (ORCPT ); Mon, 23 Jul 2018 11:32:48 -0400 Received: from foss.arm.com ([217.140.101.70]:34898 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388104AbeGWPcs (ORCPT ); Mon, 23 Jul 2018 11:32:48 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 79CE680D; Mon, 23 Jul 2018 07:31:16 -0700 (PDT) Received: from e110439-lin (e110439-lin.Emea.Arm.com [10.4.12.126]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A7D6A3F5D0; Mon, 23 Jul 2018 07:31:14 -0700 (PDT) Date: Mon, 23 Jul 2018 15:31:12 +0100 From: Patrick Bellasi To: Peter Zijlstra Cc: Alessio Balsini , linux-kernel@vger.kernel.org, Joel Fernandes , Juri Lelli , Tommaso Cucinotta , Luca Abeni , Claudio Scordino , Daniel Bristot de Oliveira , Ingo Molnar Subject: Re: [RFC PATCH] sched/deadline: sched_getattr() returns absolute dl-task information Message-ID: <20180723143112.GC2683@e110439-lin> References: <20180629120947.14579-1-alessio.balsini@gmail.com> <20180723094904.GB2494@hirez.programming.kicks-ass.net> <20180723124946.GA2683@e110439-lin> <20180723141322.GZ2458@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180723141322.GZ2458@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 23-Jul 16:13, Peter Zijlstra wrote: > On Mon, Jul 23, 2018 at 01:49:46PM +0100, Patrick Bellasi wrote: > > On 23-Jul 11:49, Peter Zijlstra wrote: > > > > [...] > > > > > > -void __getparam_dl(struct task_struct *p, struct sched_attr *attr) > > > > +void __getparam_dl(struct task_struct *p, struct sched_attr *attr, > > > > + unsigned int flags) > > > > { > > > > struct sched_dl_entity *dl_se = &p->dl; > > > > > > > > attr->sched_priority = p->rt_priority; > > > > - attr->sched_runtime = dl_se->dl_runtime; > > > > - attr->sched_deadline = dl_se->dl_deadline; > > > > + > > > > + if (flags & SCHED_GETATTR_FLAGS_DL_ABSOLUTE) { > > > > + /* > > > > + * If the task is not running, its runtime is already > > > > + * properly accounted. Otherwise, update clocks and the > > > > + * statistics for the task. > > > > + */ > > > > + if (task_running(task_rq(p), p)) { > > > > + struct rq_flags rf; > > > > + struct rq *rq; > > > > + > > > > + rq = task_rq_lock(p, &rf); > > > > + sched_clock_tick(); > > > > + update_rq_clock(rq); > > > > + task_tick_dl(rq, p, 0); > > > > > > Do we really want task_tick_dl() here, or update_curr_dl()? > > > > I think this was to cover the case of a syscall being called while the > > task is running and we are midway between two ticks... > > Sure, I know what it's there for, just saying that update_curr_dl() > would've updated the accounting as well. Calling tick stuff from !tick > context is a wee bit dodgy. Right, I think it depends on how much we want to be "precise" in closing a control loop with user-space. On Android we have ticks every 3-4ms, I'm wondering if this maximum "latency" on measuring the remaining run-time can introduce a too big error for certain applications... Alessio: you have an interesting low-latency audio use-case on hand, do you think we can tolerate a 4ms error in remaining run-time readings? [...] > > Which means we should use something like: > > > > if (flags & SCHED_GETATTR_FLAGS_DL_ABSOLUTE) { > > /* Lock the task and the RQ before any other check and upate */ > > rq = task_rq_lock(p, &rf); > > > > /* Check the task is still DL ?*/ > > > > /* Update task stats */ > > > > task_rq_unlock(rq, p, &rf); > > } > > > > right? > > Yeah, something along those lines. > > > If that's better, then we should probably even better move the > > task_rq_lock at the beginning of SYSCALL_DEFINE4(sched_getattr()) ? > > Hurm.. yes, we should probably have the has_dl_policy test under the > lock too. Which is really annoying, because this basically turns a > lockless syscall into locked one. Indeed... > Another method would be to have __getparam_dl() 'fail' and retry if it > finds !has_dl_policy() once we have the lock. That would retain the > lockless nature for all current use-cases and only incur the locking > overhead for this new case. ... right, this is actually the best solution to have a bit more guarantees for the new DL control scenarios without affecting existing ones! -- #include Patrick Bellasi