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 S934948AbbDIJv3 (ORCPT <rfc822;w@1wt.eu>);
	Thu, 9 Apr 2015 05:51:29 -0400
Received: from www.linutronix.de ([62.245.132.108]:45741 "EHLO
	Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S933221AbbDIIxX (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 9 Apr 2015 04:53:23 -0400
Date: Thu, 9 Apr 2015 10:53:40 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Ingo Molnar <mingo@kernel.org>
cc: Viresh Kumar <viresh.kumar@linaro.org>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, linaro-kernel@lists.linaro.org,
        linux-kernel@vger.kernel.org,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>
Subject: Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct
 check of the active list
In-Reply-To: <20150409062841.GB14259@gmail.com>
Message-ID: <alpine.DEB.2.11.1504091050150.3845@nanos>
References: <cover.1428039899.git.viresh.kumar@linaro.org> <b884675ccbd86069d7e9ff6b0de35760c5d6a959.1428039899.git.viresh.kumar@linaro.org> <alpine.DEB.2.11.1504082019220.3845@nanos> <20150409062841.GB14259@gmail.com>
User-Agent: Alpine 2.11 (DEB 23 2013-08-11)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-Linutronix-Spam-Score: -1.0
X-Linutronix-Spam-Level: -
X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required,  ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, 9 Apr 2015, Ingo Molnar wrote:
> Btw., does cpu_base->active_bases even make sense? hrtimer bases are 
> fundamentally percpu, and to check whether there are any pending 
> timers is a very simple check:
> 
> 	base->active->next != NULL
> 
> So I'd rather suggest taking a direct look at the head, instead of 
> calculating bit positions, masks, etc.
>
> Furthermore, we never actually use cpu_base->active_bases as a 
> 'summary' value (which is the main point of bitmasks in general), so 
> I'd remove that complication altogether.
> 
> This would speed up various hrtimer primitives like 
> hrtimer_remove()/add and simplify the code. It would be a net code 
> shrink as well.

Well. You trade a bit more code against touching cache lines to figure
out whether the clock base has active timers or not. So for a lot of
scenarios where only clock monotonic is used you touch 3 cache lines
for nothing.

I'm about to send out a patch which actually makes better use of the
active_bases field without creating a code size explosion.

Thanks,

	tglx