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 S1754329AbdEQMqm (ORCPT <rfc822;w@1wt.eu>);
        Wed, 17 May 2017 08:46:42 -0400
Received: from mail-wm0-f47.google.com ([74.125.82.47]:35138 "EHLO
        mail-wm0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1754175AbdEQMqi (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 17 May 2017 08:46:38 -0400
Date: Wed, 17 May 2017 13:46:35 +0100
From: Matt Fleming <matt@codeblueprint.co.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, tglx@linutronix.de, riel@redhat.com, hpa@zytor.com,
        efault@gmx.de, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, lvenanci@redhat.com,
        xiaolong.ye@intel.com, kitsunyan@inbox.ru, clm@fb.com
Subject: Re: hackbench vs select_idle_sibling; was: [tip:sched/core]
 sched/fair, cpumask: Export for_each_cpu_wrap()
Message-ID: <20170517124635.GB3229@codeblueprint.co.uk>
References: <20170414122005.o35me2h5nowqkxbv@hirez.programming.kicks-ass.net>
 <tip-c743f0a5c50f2fcbc628526279cfa24f3dabe182@git.kernel.org>
 <20170517105350.hk5m4h4jb6dfr65a@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170517105350.hk5m4h4jb6dfr65a@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.5.24+41 (02bc14ed1569) (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, 17 May, at 12:53:50PM, Peter Zijlstra wrote:
> On Mon, May 15, 2017 at 02:03:11AM -0700, tip-bot for Peter Zijlstra wrote:
> > sched/fair, cpumask: Export for_each_cpu_wrap()
> 
> > -static int cpumask_next_wrap(int n, const struct cpumask *mask, int start, int *wrapped)
> > -{
> 
> > -	next = find_next_bit(cpumask_bits(mask), nr_cpumask_bits, n+1);
> 
> > -}
> 
> OK, so this patch fixed an actual bug in the for_each_cpu_wrap()
> implementation. The above 'n+1' should be 'n', and the effect is that
> it'll skip over CPUs, potentially resulting in an iteration that only
> sees every other CPU (for a fully contiguous mask).
> 
> This in turn causes hackbench to further suffer from the regression
> introduced by commit:
> 
>   4c77b18cf8b7 ("sched/fair: Make select_idle_cpu() more aggressive")
> 
> So its well past time to fix this.
> 
> Where the old scheme was a cliff-edge throttle on idle scanning, this
> introduces a more gradual approach. Instead of stopping to scan
> entirely, we limit how many CPUs we scan.
> 
> Initial benchmarks show that it mostly recovers hackbench while not
> hurting anything else, except Mason's schbench, but not as bad as the
> old thing.
> 
> It also appears to recover the tbench high-end, which also suffered like
> hackbench.
> 
> I'm also hoping it will fix/preserve kitsunyan's interactivity issue.
> 
> Please test..

Tests queued up at SUSE. I'll let you know the results as soon as
they're ready -- it'll be at least a couple of days.