linux-kernel - Re: [patch 0/7] sched: change nohz idle load balancing logic to push model

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Date:	Thu, 20 May 2010 12:50:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.jf.intel.com>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	ego@...ibm.com, LKML <linux-kernel@...r.kernel.org>,
	Dominik Brodowski <linux@...inikbrodowski.net>,
	Nigel Cunningham <ncunningham@...a.org.au>,
	David Miller <davem@...emloft.net>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [patch 0/7] sched: change nohz idle load balancing logic to
 push model

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.


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