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=-2.1 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, 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 01034ECDFB8 for ; Mon, 23 Jul 2018 14:13:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AA7BB20852 for ; Mon, 23 Jul 2018 14:13:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="QxRF6YLa" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AA7BB20852 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org 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 S2388410AbeGWPPG (ORCPT ); Mon, 23 Jul 2018 11:15:06 -0400 Received: from merlin.infradead.org ([205.233.59.134]:41688 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388112AbeGWPPG (ORCPT ); Mon, 23 Jul 2018 11:15:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=+mNtInzZZTWCQUh0J0c9ts0fB5z8wjF04lz5VAF5crQ=; b=QxRF6YLa2oJ5WUj1GHqZvds/Q aTiHC9hk53YTzBP9eetBe1rjegohDDm2H4vl1EwfjVqnVTDyNZ+aBLyUk5TdVZguhDqFgKLQEj5nW 1LJrF3oSYAU7YkJdtnnFFBCsT7TsVfHj+NLEwJrDeEkxFVv6b01ZiLHxJAt3Re4/KeLAD5GYMfI/Q lKGaN3yyxUye2QxBTA5wLJKY5fuY0F9rcl8d4s06+y9WZI/F984x3WcZE5EL3ZMf5dm7gcNdFYRnt y0WQZo/R1lyZLVCNFKx5OWqG+8phdHOqEfjBHaAejuliSYacrzs0TNNKLP6qg5ltBT8BJEP+JarNI 8zhBapLRw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fhbaW-0007Rs-3Z; Mon, 23 Jul 2018 14:13:24 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id AD6D920275F47; Mon, 23 Jul 2018 16:13:22 +0200 (CEST) Date: Mon, 23 Jul 2018 16:13:22 +0200 From: Peter Zijlstra To: Patrick Bellasi 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: <20180723141322.GZ2458@hirez.programming.kicks-ass.net> References: <20180629120947.14579-1-alessio.balsini@gmail.com> <20180723094904.GB2494@hirez.programming.kicks-ass.net> <20180723124946.GA2683@e110439-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180723124946.GA2683@e110439-lin> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > Also, who says the task still is dl ? :-) > > Good point, but what should be the rule in general for these cases? > > We already have: > > SYSCALL_DEFINE4(sched_getattr()) > .... > if (task_has_dl_policy(p)) > __getparam_dl(p, &attr); > > which is also potentially racy, isn't it? Yes, but only in so far as that the whole syscall is racy per-definition. EVen if we'd lock the rq and get the absolute accurate values, everything can change the moment we release the locks and return to userspace again. > Or just make the syscall return the most updated metrics for all the > scheduling classes since we cannot grant the user anything about what > the task will be once we return to userspace? This. > > > + task_rq_unlock(rq, p, &rf); > > > + } > > > + > > > + /* > > > + * If the task is throttled, this value could be negative, > > > + * but sched_runtime is unsigned. > > > + */ > > > + attr->sched_runtime = dl_se->runtime <= 0 ? 0 : dl_se->runtime; > > > + attr->sched_deadline = dl_se->deadline; > > > > This is all very racy.. > > > > Even if the task wasn't running when you did the task_running() test, it > > could be running now. And if it was running, it might not be running > > anymore by the time you've acquired the rq->lock. > > 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. 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.