Rescheduling Interrupts, Tweaking the Scheduler, and bug #177895

[Ben Collins]

On Fri, 2008-04-04 at 12:32 +0100, Colin Ian King wrote:
> Hi all,
> 
> Some apps on duo core machines seem to be generating a lot of kernel
> wakeups (for example amarok during mp3 playback) which in turn generates
> a lot of "Rescheduling Interrupts" as the scheduler tries to re-balance
> core useage. 
> 
> A lot of these rescheduling interrupts seem legitimate wake-ups, for
> example amarok waking up the 2nd core to make X re-render on one core
> while another core busy doing mp3 decode and playback. However the
> rescheduling interrupts do look very high on an nearly idle system (e.g.
> just running amarok!) when examining a running system with tools such as
> powertop.
> 
> Alessio has been helpful to find an upstream patch that reduces the
> heavy handed rescheduling interrupts when the system is idle on a multi
> core system. namely upstream commit
> 33b0c4217dcd67b788318c3192a2912b530e4eef which tweaks the multi-core
> scheduler intialiser flags in include/linux/topology.h to be less
> aggressive for IDLE wake ups.
> 
> With the fix, we do see a reduction in rescheduling interrupts - that
> is, sleeping cores are woken up less by kernel IPI events, however the
> CPU C0..C3 residency does change:
> 
> 		"Fixed" Kernel		Current Kernel
> Rescheduling	   
> Interrupts/sec	~210-220		~240-250
> 
> C0 residency	37%			23%		
> C1   ""		 0%			0%	
> C2   ""		 0%			0%
> C3   ""		63%			28%
> 
> Power (Watts)	30.1			27.4
> 
> Figures were obtained from powertop running for 8 minutes with average
> of 10 second samples.
> 
> So... it appears that reducing the "Rescheduling Interrupts" by tweaking
> the current scheduler reduces the overall time the processor core is in
> the lowest C3 state, and increases the busy C0 state residency, hence
> increasing the overall power usage.
> 
> With this in mind, I think I will no longer pursue any more fixes to the
> scheduler to reduced the "Scheduling Interrupts" as it is detrimental to
> power consumption for laptop users et al.
> 
> There are a heap of other more significant changes upstream in the
> scheduler which may improve things generally but they are far too
> intrusive to make so late in the Hardy release cycle.
> 
> I will therefore close bug 177895 as "Won't Fix" - and add some notes to
> explain why.
> 
> Is this OK?

Excellent investigation into this bug. I'm in agreement with your
evaluation and suggested handling of the bug report. +1 from me.

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