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

[Fushen Chen]

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 at kernel.crashing.org> wrote:

> 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.
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ozlabs.org/pipermail/linuxppc-dev/attachments/20111213/8944a5f2/attachment.html>