From patchwork Mon Feb 18 12:39:07 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Srivatsa S. Bhat" X-Patchwork-Id: 221287 Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from ozlabs.org (localhost [IPv6:::1]) by ozlabs.org (Postfix) with ESMTP id E20A92C04D3 for ; Mon, 18 Feb 2013 23:46:17 +1100 (EST) Received: from e28smtp03.in.ibm.com (e28smtp03.in.ibm.com [122.248.162.3]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "e28smtp03.in.ibm.com", Issuer "GeoTrust SSL CA" (not verified)) by ozlabs.org (Postfix) with ESMTPS id D834C2C02A2 for ; Mon, 18 Feb 2013 23:41:15 +1100 (EST) Received: from /spool/local by e28smtp03.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 18 Feb 2013 18:08:52 +0530 Received: from d28dlp03.in.ibm.com (9.184.220.128) by e28smtp03.in.ibm.com (192.168.1.133) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Mon, 18 Feb 2013 18:08:49 +0530 Received: from d28relay02.in.ibm.com (d28relay02.in.ibm.com [9.184.220.59]) by d28dlp03.in.ibm.com (Postfix) with ESMTP id 980D31258051 for ; Mon, 18 Feb 2013 18:11:55 +0530 (IST) Received: from d28av04.in.ibm.com (d28av04.in.ibm.com [9.184.220.66]) by d28relay02.in.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r1ICf7JN30081146 for ; Mon, 18 Feb 2013 18:11:08 +0530 Received: from d28av04.in.ibm.com (loopback [127.0.0.1]) by d28av04.in.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r1ICf5Eo018691 for ; Mon, 18 Feb 2013 23:41:08 +1100 Received: from srivatsabhat.in.ibm.com (srivatsabhat.in.ibm.com [9.124.35.204] (may be forged)) by d28av04.in.ibm.com (8.14.4/8.13.1/NCO v10.0 AVin) with ESMTP id r1ICf4Dw018633; Mon, 18 Feb 2013 23:41:04 +1100 From: "Srivatsa S. Bhat" Subject: [PATCH v6 06/46] percpu_rwlock: Rearrange the read-lock code to fastpath nested percpu readers To: tglx@linutronix.de, peterz@infradead.org, tj@kernel.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org Date: Mon, 18 Feb 2013 18:09:07 +0530 Message-ID: <20130218123907.26245.37550.stgit@srivatsabhat.in.ibm.com> In-Reply-To: <20130218123714.26245.61816.stgit@srivatsabhat.in.ibm.com> References: <20130218123714.26245.61816.stgit@srivatsabhat.in.ibm.com> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13021812-3864-0000-0000-000006E0F203 Cc: linux-arch@vger.kernel.org, linux@arm.linux.org.uk, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, fweisbec@gmail.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, rostedt@goodmis.org, xiaoguangrong@linux.vnet.ibm.com, rjw@sisk.pl, sbw@mit.edu, wangyun@linux.vnet.ibm.com, srivatsa.bhat@linux.vnet.ibm.com, netdev@vger.kernel.org, vincent.guittot@linaro.org, walken@google.com, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+patchwork-incoming=ozlabs.org@lists.ozlabs.org Sender: "Linuxppc-dev" If we are dealing with a nester percpu reader, we can optimize away quite a few costly operations. Improve that fastpath further, by avoiding the unnecessary addition and subtraction of 'READER_PRESENT' to reader_refcnt, by rearranging the code a bit. Signed-off-by: Srivatsa S. Bhat --- lib/percpu-rwlock.c | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/lib/percpu-rwlock.c b/lib/percpu-rwlock.c index ce7e440..ed36531 100644 --- a/lib/percpu-rwlock.c +++ b/lib/percpu-rwlock.c @@ -80,23 +80,21 @@ void percpu_read_lock_irqsafe(struct percpu_rwlock *pcpu_rwlock) preempt_disable(); /* - * Let the writer know that a reader is active, even before we choose - * our reader-side synchronization scheme. - */ - this_cpu_add(pcpu_rwlock->rw_state->reader_refcnt, READER_PRESENT); - - /* * If we are already using per-cpu refcounts, it is not safe to switch * the synchronization scheme. So continue using the refcounts. */ if (reader_uses_percpu_refcnt(pcpu_rwlock)) { this_cpu_inc(pcpu_rwlock->rw_state->reader_refcnt); - this_cpu_sub(pcpu_rwlock->rw_state->reader_refcnt, - READER_PRESENT); return; } /* + * Let the writer know that a reader is active, even before we choose + * our reader-side synchronization scheme. + */ + this_cpu_add(pcpu_rwlock->rw_state->reader_refcnt, READER_PRESENT); + + /* * The write to 'reader_refcnt' must be visible before we read * 'writer_signal'. */