From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761626AbYDKRlu (ORCPT ); Fri, 11 Apr 2008 13:41:50 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760424AbYDKRln (ORCPT ); Fri, 11 Apr 2008 13:41:43 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:45478 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760086AbYDKRlm (ORCPT ); Fri, 11 Apr 2008 13:41:42 -0400 Subject: Re: CFS rq lock question From: Peter Zijlstra To: Dan Upton Cc: linux-kernel@vger.kernel.org, Ingo Molnar In-Reply-To: <1207935421.7524.3.camel@twins> References: <1207935421.7524.3.camel@twins> Content-Type: text/plain Date: Fri, 11 Apr 2008 19:41:40 +0200 Message-Id: <1207935700.7524.4.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2008-04-11 at 19:37 +0200, Peter Zijlstra wrote: > On Fri, 2008-04-11 at 13:21 -0400, Dan Upton wrote: > > I'm poking around with some scheduler stuff, and there's something I'm > > not clear on for the CFS runqueue locks. The comments before > > __load_balance_iterator(...) in sched_fair.c suggests things can be > > dequeued even though the runqueue lock is held. Can things also be > > added to the queue while the lock is held? (Also, either way, what's > > the rationale that dequeueing is a safe procedure when somebody else > > holds a lock?) > > /* > * Load-balancing iterator. Note: while the runqueue stays locked > * during the whole iteration, the current task might be > * dequeued so the iterator has to be dequeue-safe. Here we > * achieve that by always pre-iterating before returning > * the current task: > */ > > I don't think this comment is correct, but if it were, it would only > apply to rq->curr, not for any enqueue/dequeue. > D'oh, I'm silly.. the task can be dequeued because we move it to another cpu.