Rescheduling Interrupts, Tweaking the Scheduler, and bug #177895

[Matt Price]

On Fri, 2008-04-04 at 09:03 -0400, Ben Collins wrote:
> 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.
> 
just wanted to say as a user troubled by this question how great it is
to have such a helpful explanation of the issue -- maybe put the whole
email in the bug report?  it's really thoughtful.  

thanks,
matt


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> 
-- 
Matt Price
matt.price at utoronto.ca