Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966313AbbBCRQ5 (ORCPT ); Tue, 3 Feb 2015 12:16:57 -0500 Received: from mga03.intel.com ([134.134.136.65]:59137 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965829AbbBCRQy (ORCPT ); Tue, 3 Feb 2015 12:16:54 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.09,513,1418112000"; d="scan'208";a="646759917" Message-ID: <1422983812.9530.43.camel@schen9-desk2.jf.intel.com> Subject: Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners From: Tim Chen To: Davidlohr Bueso Cc: Peter Zijlstra , Ingo Molnar , "Paul E. McKenney" , Jason Low , Michel Lespinasse , linux-kernel@vger.kernel.org Date: Tue, 03 Feb 2015 09:16:52 -0800 In-Reply-To: <1422671289.28351.1.camel@stgolabs.net> References: <1422609267-15102-1-git-send-email-dave@stgolabs.net> <1422609267-15102-5-git-send-email-dave@stgolabs.net> <1422669098.9530.33.camel@schen9-desk2.jf.intel.com> <1422671289.28351.1.camel@stgolabs.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.8.5 (3.8.5-2.fc19) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1359 Lines: 38 > > > > > > + if (READ_ONCE(sem->owner)) > > > + return true; /* new owner, continue spinning */ > > > + > > > > Do you have some comparison data of whether it is more advantageous > > to continue spinning when owner changes? After the above change, > > rwsem will behave more like a spin lock for write lock and > > will keep spinning when the lock changes ownership. > > But recall we still abort when need_resched, so the spinning isn't > infinite. Never has been. > > > Now during heavy > > lock contention, if we don't continue spinning and sleep, we may use the > > clock cycles for actually running other threads. > > Under heavy contention, time spinning will force us to ultimately block > anyway. The question is under heavy contention, if we are going to block anyway, won't it be more advantageous not to continue spinning so we can use the cycles for useful task? The original code assumes that if the lock has switched owner, then we are under heavy contention and we can stop spinning and block. I think it'll be useful to have some data comparing the two behaviors. Thanks. Tim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/