linux-kernel - NO_HZ and cpu monitoring tools

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Message-ID: <20071014234242.GB24519@kryten>
Date:	Sun, 14 Oct 2007 18:42:43 -0500
From:	Anton Blanchard <anton@...ba.org>
To:	mingo@...e.hu, tglx@...utronix.de
Cc:	linux-kernel@...r.kernel.org
Subject: NO_HZ and cpu monitoring tools


Hi,

When using a NO_HZ kernel on ppc64, I noticed top gives some interesting
results:

Cpu0  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu1  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu2  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu3  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu4  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu5  :  1.1%us,  0.0%sy,  0.0%ni, 98.9%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu6  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st
Cpu7  :  0.0%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si, 0.0%st

Notice how only 2 cpus report idle time. Im guessing this happens if 
a core sleeps for longer than the update period in top. Where should
this be fixed?

It would be possible for the proc read method to add in the right number
of idle jiffies, or top could just assume no increment means 100% idle.

Anton
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