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 S1755081Ab0ETKu5 (ORCPT <rfc822;w@1wt.eu>);
	Thu, 20 May 2010 06:50:57 -0400
Received: from bombadil.infradead.org ([18.85.46.34]:49839 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752275Ab0ETKuz convert rfc822-to-8bit (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 20 May 2010 06:50:55 -0400
Subject: Re: [patch 0/7] sched: change nohz idle load balancing logic to
 push model
From: Peter Zijlstra <peterz@infradead.org>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       Arjan van de Ven <arjan@linux.jf.intel.com>,
       Venkatesh Pallipadi <venki@google.com>,
       Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>, ego@in.ibm.com,
       LKML <linux-kernel@vger.kernel.org>,
       Dominik Brodowski <linux@dominikbrodowski.net>,
       Nigel Cunningham <ncunningham@crca.org.au>,
       David Miller <davem@davemloft.net>, Jens Axboe <jens.axboe@oracle.com>
In-Reply-To: <20100517182726.089700767@sbs-t61.sc.intel.com>
References: <20100517182726.089700767@sbs-t61.sc.intel.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8BIT
Date: Thu, 20 May 2010 12:50:21 +0200
Message-ID: <1274352621.5605.14017.camel@twins>
Mime-Version: 1.0
X-Mailer: Evolution 2.28.3 
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, 2010-05-17 at 11:27 -0700, Suresh Siddha wrote:
> This is an updated version of patchset which is posted earlier at
> http://lkml.org/lkml/2009/12/10/470
> 
> Description:
> Existing nohz idle load balance logic uses the pull model, with one
> idle load balancer CPU nominated on any partially idle system and that
> balancer CPU not going into nohz mode. With the periodic tick, the
> balancer does the idle balancing on behalf of all the CPUs in nohz mode.
> 
> This is not very optimal and has few issues:
> * the balancer will continue to have periodic ticks and wakeup
>   frequently (HZ rate), even though it may not have any rebalancing to do on
>   behalf of any of the idle CPUs.
> * On x86 and CPUs that have APIC timer stoppage on idle CPUs, this periodic
>   wakeup can result in an additional interrupt on a CPU doing the timer
>   broadcast.
> 
> The alternative is to have a push model, where all idle CPUs can enter nohz
> mode and any busy CPU kicks one of the idle CPUs to take care of idle
> balancing on behalf of a group of idle CPUs.
> 
> Following patches switches idle load balancer to this push approach.
> 
> Updates from the previous version:
> 
> * Busy CPU uses send_remote_softirq() for invoking SCHED_SOFTIRQ on the
>   idle load balancing cpu, which does the load balancing on behalf of
>   all the idle CPUs.
> 
> * Dropped the per NUMA node nohz load balancing as it doesn't detect
>   certain imbalance scenarios. This will be addressed later.

Looks good.

I think we want to keep init_remote_softirq_csd() and a function that
directly triggers the relevant softirq and make the networking code and
the block layer use that if possible -- and axe the rest of the
send_remote_softirq() infrastructure.

Also, I think it makes sense to fold patches 4-6.