Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757148Ab1BJVNi (ORCPT ); Thu, 10 Feb 2011 16:13:38 -0500 Received: from relay2.sgi.com ([192.48.179.30]:32777 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1757118Ab1BJVNh (ORCPT ); Thu, 10 Feb 2011 16:13:37 -0500 Date: Thu, 10 Feb 2011 15:12:23 -0600 From: Jack Steiner To: David Miller Cc: mingo@elte.hu, raz@scalemp.com, linux-kernel@vger.kernel.org, mingo@redhat.com, a.p.zijlstra@chello.nl, efault@gmx.de, cpw@sgi.com, travis@sgi.com, tglx@linutronix.de, hpa@zytor.com Subject: Re: [BUG] soft lockup while booting machine with more than 700 cores Message-ID: <20110210211223.GB10757@sgi.com> References: <1297236453.2756.9.camel@raz.scalemp.com> <20110210123937.GD26094@elte.hu> <20110210205648.GA10341@sgi.com> <20110210.130325.112603217.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110210.130325.112603217.davem@davemloft.net> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1725 Lines: 36 On Thu, Feb 10, 2011 at 01:03:25PM -0800, David Miller wrote: > From: Jack Steiner > Date: Thu, 10 Feb 2011 14:56:48 -0600 > > > We also noticed that the rebalance_domains() code references many per-cpu > > run queue structures. All of the structures have identical offsets relative > > to the size of a cache leaf. The result is that all index into the same lines in the > > L3 caches. That causes many evictions. We tried an experimental to > > stride the run queues at 128 byte offsets. That helped in some cases but the > > results were mixed. We are still experimenting with the patch. > > I think chasing after cache alignment issues misses the point entirely. > > The core issue is that rebalance_domains() is insanely expensive, by > design. It's complexity is N factorial for the idle non-HZ cpu that is > selected to balance every single domain. > > A statistic datastructure that is approximately 128 bytes in size is > repopulated N! times each time this global rebalance thing runs. > > I've been seeing rebalance_domains() in my perf top output on 128 cpu > machines for several years now. Even on an otherwise idle machine, > the system churns in thus code path endlessly. Completely agree! Idle rebalancing is also a big problem. We've seen significant improvements on large systems in network thruput by disabling IDLE load balancing for the higher (2 & 3) scheduling domains. This is not a real fix but points to a problem. --- jack -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/