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

On Wed, 14 Dec 2011, Benjamin Herrenschmidt wrote:

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

quoted

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 ????

Thanks,

	tglx