Re: CONFIG_NO_HZ added too much idle time in /proc/stat during throughput test.

This is 2.6.32, but I think 2.6.36 is the same.
Thanks,
Fushen

On Tue, Dec 13, 2011 at 3:34 PM, Benjamin Herrenschmidt <
benh@kernel.crashing.org> wrote:

On Wed, 2011-12-14 at 00:28 +0100, Thomas Gleixner wrote:

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On Wed, 14 Dec 2011, Benjamin Herrenschmidt wrote:

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On Tue, 2011-12-13 at 12:42 -0800, Fushen Chen wrote:

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On APM82181,  "vmstat" (/proc/stat)  doesn't show correct idle
percent, if kernel enables "CONFIG_NO_HZ" (Tickless System / Dynamic
Tick).

When I run wireless throughput test with heavy traffic, "vmstat"

shows

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very high idle percent while "oprofile" shows very low idle percent.
During the test, the system is idle, but network traffic uses a lot

of

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hard IRQ and soft-irq time. "vmstat" would have the correct stats if
account_idle_ticks(ticks) in kernel/time/tick-sched.c doesn't add

more

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idle time in "vmstat". In the same test, if I disable "CONFIG_NO_HZ"
in kernel, idle percent in "vmstat" and "oprofile" would match.

My APM82181 kernel configuration is "CONFIG_NO_HZ",

"CONFIG_HZ_250=y",

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"CONFIG_HZ=250", and "CONFIG_HIGH_RES_TIMERS".

My question is that if kernel enables "CONFIG_NO_HZ", how would

kernel

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report correct stats.

Hi Thomas ! Any idea what we're doing wrong ? :-)

Not really, that had been an issue before and had been fixed. Peter ????

Fusen, what kernel version is this ?

Cheers,
Ben.