From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754840AbbDIHJ3 (ORCPT ); Thu, 9 Apr 2015 03:09:29 -0400 Received: from mail-wg0-f43.google.com ([74.125.82.43]:35400 "EHLO mail-wg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753314AbbDIHJW (ORCPT ); Thu, 9 Apr 2015 03:09:22 -0400 Date: Thu, 9 Apr 2015 09:09:17 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Thomas Gleixner , Viresh Kumar , Ingo Molnar , linaro-kernel@lists.linaro.org, linux-kernel@vger.kernel.org, Preeti U Murthy Subject: Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct check of the active list Message-ID: <20150409070917.GF14259@gmail.com> References: <20150409062841.GB14259@gmail.com> <20150409065730.GK27490@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150409065730.GK27490@worktop.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Thu, Apr 09, 2015 at 08:28:41AM +0200, 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 > > > > Yeah, that's 3 pointer dereferences from cpu_base, iow you traded a > single bit test on an already loaded word for 3 potential cacheline > misses. But the clock bases are not aligned to cachelines, and we have 4 of them. So in practice when we access one, we'll load the next one anyway. Furthermore the simplification is measurable, and a fair bit of it is in various fast paths. I'd rather trade a bit of a cacheline footprint for less overall complexity and faster code. Thanks, Ingo