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

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 14 Dec 2011 10:24:10 +1100

On Tue, 2011-12-13 at 12:42 -0800, Fushen Chen wrote:
> 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
> 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
> 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
> 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",
> "CONFIG_HZ=250", and "CONFIG_HIGH_RES_TIMERS".
> 
> My question is that if kernel enables "CONFIG_NO_HZ", how would kernel
> report correct stats.

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

Cheers,
Ben.

> Thanks,
> Fushen
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