From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754668AbbINOFM (ORCPT <rfc822;w@1wt.eu>);
	Mon, 14 Sep 2015 10:05:12 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:54372 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751938AbbINOFK (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 14 Sep 2015 10:05:10 -0400
Date: Mon, 14 Sep 2015 16:04:56 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>, Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt
Message-ID: <20150914140456.GU18489@twins.programming.kicks-ass.net>
References: <1441996658-62854-1-git-send-email-Waiman.Long@hpe.com>
 <1441996658-62854-6-git-send-email-Waiman.Long@hpe.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1441996658-62854-6-git-send-email-Waiman.Long@hpe.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
> This patch allows one attempt for the lock waiter to steal the lock
> when entering the PV slowpath.  This helps to reduce the performance
> penalty caused by lock waiter preemption while not having much of
> the downsides of a real unfair lock.


> @@ -416,7 +414,8 @@ queue:
>  	 * does not imply a full barrier.
>  	 *
>  	 */

If it really were once, like the Changelog says it is, then you could
have simply added:

	if (pv_try_steal_lock(...))
		goto release;

here, and not wrecked pv_wait_head() like you did. Note that if you do
it like this, you also do not need to play games with the hash, because
you'll never get into that situation.

> -	pv_wait_head(lock, node);
> +	if (pv_wait_head_and_lock(lock, node, tail))
> +		goto release;
>  	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
>  		cpu_relax();
>