From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753043AbeDKSBK (ORCPT ); Wed, 11 Apr 2018 14:01:10 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:52054 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752584AbeDKSBG (ORCPT ); Wed, 11 Apr 2018 14:01:06 -0400 From: Will Deacon To: linux-kernel@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org, peterz@infradead.org, mingo@kernel.org, boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com, longman@redhat.com, Will Deacon Subject: [PATCH v2 03/13] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound Date: Wed, 11 Apr 2018 19:01:10 +0100 Message-Id: <1523469680-17699-4-git-send-email-will.deacon@arm.com> X-Mailer: git-send-email 2.1.4 In-Reply-To: <1523469680-17699-1-git-send-email-will.deacon@arm.com> References: <1523469680-17699-1-git-send-email-will.deacon@arm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On x86, atomic_cond_read_relaxed will busy-wait with a cpu_relax() loop, so it is desirable to increase the number of times we spin on the qspinlock lockword when it is found to be transitioning from pending to locked. According to Waiman Long: | Ideally, the spinning times should be at least a few times the typical | cacheline load time from memory which I think can be down to 100ns or | so for each cacheline load with the newest systems or up to several | hundreds ns for older systems. which in his benchmarking corresponded to 512 iterations. Cc: Peter Zijlstra Cc: Ingo Molnar Suggested-by: Waiman Long Signed-off-by: Will Deacon --- arch/x86/include/asm/qspinlock.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h index 5e16b5d40d32..2f09915f4aa4 100644 --- a/arch/x86/include/asm/qspinlock.h +++ b/arch/x86/include/asm/qspinlock.h @@ -7,6 +7,8 @@ #include #include +#define _Q_PENDING_LOOPS (1 << 9) + #define queued_spin_unlock queued_spin_unlock /** * queued_spin_unlock - release a queued spinlock -- 2.1.4