linux-kernel - Re: CPU scheduler weirdness?

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Message-ID: <20090813155812.GA15714@rhlx01.hs-esslingen.de>
Date:	Thu, 13 Aug 2009 17:58:12 +0200
From:	Andreas Mohr <andi@...as.de>
To:	Marton Balint <cus@...ekas.hu>
Cc:	Andreas Mohr <andi@...as.de>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, peterz@...radead.org
Subject: Re: CPU scheduler weirdness?

Hi,

On Thu, Aug 13, 2009 at 05:39:10PM +0200, Marton Balint wrote:
>>> Does anybody have any idea what can cause this?
>>
>> /sys/devices/system/cpu/sched_smt_power_savings , perhaps?
>
> Thanks for the tip, tuning the sched_mc_power_savings setting helped! The 
> original value of it was 0, but after setting it to 1, the two  
> cpu-intensive processes got scheduled to different CPU cores, as 
> expected.

Heh, I did expect it to not help, and indeed that thing helping in this
way points to a... BUG, plain and simple.

http://lwn.net/Articles/297306/
lists possible settings as

"
The power savings and performance of the given workload in an under
utilised system can be controlled by setting values of 0, 1 or 2 to
/sys/devices/system/cpu/sched_mc_power_savings with 0 being highest
performance and least power savings and level 2 indicating maximum
power savings even at the cost of slight performance degradation.
"

which is exactly opposite to what I'd have expected to be normal,
unconfigured behaviour in your case.

> Setting it back to 0 casused the two cpu-intensive processes to run on  
> the same CPU again. So I guess I will just set it to 1 after booting the  
> system.

...which would indicate a level=1 or level=2 (maximum powersaving)
behaviour. Something either seems reversed or really weird.
But it could just be opaque if correct behaviour due to a much more complex
load balancing algo in the scheduler or so.

Comments, anyone?

Andreas Mohr
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