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

[Benjamin Herrenschmidt]

On Wed, 2011-12-14 at 00:28 +0100, Thomas Gleixner wrote:
> On Wed, 14 Dec 2011, Benjamin Herrenschmidt wrote:
> 
> > 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 ? :-)
> 
> Not really, that had been an issue before and had been fixed. Peter ????

Fusen, what kernel version is this ?

Cheers,
Ben.