From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932600AbbDIIKp (ORCPT ); Thu, 9 Apr 2015 04:10:45 -0400 Received: from mail-wg0-f41.google.com ([74.125.82.41]:35397 "EHLO mail-wg0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751949AbbDIIKg (ORCPT ); Thu, 9 Apr 2015 04:10:36 -0400 Date: Thu, 9 Apr 2015 10:10:31 +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: <20150409081031.GA4842@gmail.com> References: <20150409062841.GB14259@gmail.com> <20150409065730.GK27490@worktop.programming.kicks-ass.net> <20150409070917.GF14259@gmail.com> <20150409080332.GL5029@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150409080332.GL5029@twins.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 09:09:17AM +0200, Ingo Molnar wrote: > > > > * 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. > > $ pahole -C hrtimer_clock_base defconfig-build/kernel/time/timer.o > struct hrtimer_clock_base { > struct hrtimer_cpu_base * cpu_base; /* 0 8 */ > int index; /* 8 4 */ > clockid_t clockid; /* 12 4 */ > struct timerqueue_head active; /* 16 16 */ > ktime_t resolution; /* 32 8 */ > ktime_t (*get_time)(void); /* 40 8 */ > ktime_t softirq_time; /* 48 8 */ > ktime_t offset; /* 56 8 */ > /* --- cacheline 1 boundary (64 bytes) --- */ > > /* size: 64, cachelines: 1, members: 8 */ > }; > > They _should_ be aligned :-) Maybe, but they aren't currently - and aligning them has costs as well. > > 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. > > cacheline misses hurt a lot, and the bitmask isn't really complex. See my other mail: in practice we already dirty all of these cachelines in the hrtimer irq... Thanks, Ingo