linux-kernel - Re: [RFC] Patch: dynticks: idle load balancing

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Date:	Wed, 7 Feb 2007 14:19:19 -0800
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	mingo@...e.hu
Cc:	Ingo Molnar <mingo@...e.hu>, vatsa@...ibm.com, clameter@....com,
	tglx@...utronix.de, arjan@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] Patch: dynticks: idle load balancing

On Tue, Jan 30, 2007 at 01:57:09PM -0800, Siddha, Suresh B wrote:
> Please let me know if you still see this issue with the latest -rt kernel.
> 
> On Tue, Jan 16, 2007 at 12:35:05PM +0100, Ingo Molnar wrote:
> > on the latest -rt kernel, when the dynticks load-balancer is enabled, 
> > then a dual-core Core2 Duo test-system increases its irq rate from the 
> > normal 15/17 per second to 300-400/sec - on a completely idle system(!). 
> > Any idea what's going on? I'll disable the load balancer for now.

Ok. got time to look into this.

The answer is simple. load_balancing in the recent kernels is happening
using SCHED_SOFTIRQ and in -rt tree that happens not in the idle process
context but in the context of softirqd for SCHED_SOFTIRQ.

This breaks the dynticks load balancer and also the regular idle load balancing
too :(

Am on to fixing the problem now :)

thanks,
suresh
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