From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752011AbcGNSht (ORCPT ); Thu, 14 Jul 2016 14:37:49 -0400 Received: from merlin.infradead.org ([205.233.59.134]:46470 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751160AbcGNShr (ORCPT ); Thu, 14 Jul 2016 14:37:47 -0400 Date: Thu, 14 Jul 2016 20:37:43 +0200 From: Peter Zijlstra To: mingo@kernel.org, linux-kernel@vger.kernel.org Cc: oleg@redhat.com, tj@kernel.org, paulmck@linux.vnet.ibm.com, john.stultz@linaro.org, dimitrysh@google.com, romlem@google.com, ccross@google.com, tkjos@google.com Subject: Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob Message-ID: <20160714183743.GK30154@twins.programming.kicks-ass.net> References: <20160714182545.786693675@infradead.org> <20160714183022.336211504@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160714183022.336211504@infradead.org> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 14, 2016 at 08:25:47PM +0200, Peter Zijlstra wrote: > @@ -19,6 +20,13 @@ int __percpu_init_rwsem(struct percpu_rw > > /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ > rcu_sync_init(&sem->rss, RCU_SCHED_SYNC); > + if (bias == PERCPU_RWSEM_WRITER) { > + /* > + * Disable rcu_sync() and force slow path. > + */ > + sem->rss.gp_count++; > + sem->rss.gp_state = !0; > + } > __init_rwsem(&sem->rw_sem, name, rwsem_key); > init_waitqueue_head(&sem->writer); > sem->state = readers_slow; So this seemed like a better deal than calling rcu_sync_enter(), because that would still incur a (pointless) synchronize_sched() at init time and people do tend to complain about things like that.