Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754391AbZCHUSY (ORCPT ); Sun, 8 Mar 2009 16:18:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753800AbZCHUSQ (ORCPT ); Sun, 8 Mar 2009 16:18:16 -0400 Received: from support.balabit.hu ([195.70.41.86]:49249 "EHLO lists.balabit.hu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753733AbZCHUSP (ORCPT ); Sun, 8 Mar 2009 16:18:15 -0400 Subject: Re: scheduler oddity [bug?] From: Balazs Scheidler To: linux-kernel@vger.kernel.org In-Reply-To: <1236451624.16726.32.camel@bzorp.balabit> References: <1236448069.16726.21.camel@bzorp.balabit> <1236451624.16726.32.camel@bzorp.balabit> Content-Type: text/plain Date: Sun, 08 Mar 2009 20:45:24 +0100 Message-Id: <1236541524.19045.6.camel@bzorp.balabit> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3241 Lines: 88 On Sat, 2009-03-07 at 19:47 +0100, Balazs Scheidler wrote: > On Sat, 2009-03-07 at 18:47 +0100, Balazs Scheidler wrote: > > Hi, > > > > I've tested this on 3 computers and each showed the same symptoms: > > * quad core Opteron, running Ubuntu kernel 2.6.27-13.29 > > * Core 2 Duo, running Ubuntu kernel 2.6.27-11.27 > > * Dual Core Opteron, Debian backports.org kernel 2.6.26-13~bpo40+1 > > > > Is this a bug, or a feature? > > > > One new interesting information: I've retested with a 2.6.22 based > kernel, and it still works there, setting the CPU affinity does not > change the performance of the test program and mpstat nicely shows that > 2 cores are working, not just one. > > Maybe this is CFS related? That was merged for 2.6.23 IIRC. > > Also, I tried changing various scheduler knobs > in /proc/sys/kernel/sched_* but they didn't help. I've tried to change > these: > > * sched_migration_cost: changed from the default 500000 to 100000 and > then 10000 but neither helped. > * sched_nr_migrate: increased it to 64, but again nothing > > I'm starting to think that this is a regression that may or may not be > related to CFS. > > I don't have a box where I could bisect on, but the test program makes > the problem quite obvious. Some more test results: Latest tree from Linus seems to work, at least the program runs on both cores as it should. I bisected the patch that changed behaviour, and I've found this: commit 38736f475071b80b66be28af7b44c854073699cc Author: Gautham R Shenoy Date: Sat Sep 6 14:50:23 2008 +0530 sched: fix __load_balance_iterator() for cfq with only one task The __load_balance_iterator() returns a NULL when there's only one sched_entity which is a task. It is caused by the following code-path. /* Skip over entities that are not tasks */ do { se = list_entry(next, struct sched_entity, group_node); next = next->next; } while (next != &cfs_rq->tasks && !entity_is_task(se)); if (next == &cfs_rq->tasks) return NULL; ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This will return NULL even when se is a task. As a side-effect, there was a regression in sched_mc behavior since 2.6.25, since iter_move_one_task() when it calls load_balance_start_fair(), would not get any tasks to move! Fix this by checking if the last entity was a task or not. Signed-off-by: Gautham R Shenoy Acked-by: Peter Zijlstra Signed-off-by: Ingo Molnar This patch was integrated for 2.6.28. With the above patch, my test program uses two cores as it should. I could only test this in a virtual machine so I don't know exact performance metrics, but I'll test 2.6.27 + plus this patch on a real box tomorrow to see if this was the culprit. I'm not sure if this is related to the avg_overlap discussion (which I honestly don't really understand :) -- Bazsi -- 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/