Re: 1.6ms jitter in rtc_wakeup (Re: 2.6.14-rc4-rt1)

From: Ingo Molnar
Date: Thu Oct 13 2005 - 22:45:26 EST

* Esben Nielsen <simlo@xxxxxxxxxx> wrote:

> I set up rtc_wakeup and got a jitter up 1.6ms!
> It came when I cd'en into a nfs-mount and typed ls.

> ls-11239 0Dn.. 4us : profile_hit (__schedule)
> ls-11239 0Dn.1 4us : sched_clock (__schedule)
> ls-11239 0Dn.1 5us : check_tsc_unstable (sched_clock)
> ls-11239 0Dn.1 5us : tsc_read_c3_time (sched_clock)
> IRQ 8-775 0D..2 6us : __switch_to (__schedule)
> IRQ 8-775 0D..2 7us!: __schedule <ls-11239> (75 0)
> IRQ 8-775 0...1 1594us : trace_stop_sched_switched (__schedule)
> IRQ 8-775 0D..2 1594us : trace_stop_sched_switched <IRQ 8-775> (0 0)
> IRQ 8-775 0D..2 1595us : trace_stop_sched_switched (__schedule)

ouch! This very much looks like a hardware induced latency, because the
codepath from those two __schedule points is extremely short and there
is no loop there. Have you tested this particular box before too? If
not, can you reproduce this latency with older versions of -rt too on
the same box, or is this completely new? Occasionally there are boxes
that show clear signs of hardware latencies - there's little the kernel
can do about those.

Wild shot in the dark: are there any power-saving modes enabled on the
box? Another shot in the dark: can you trigger these latencies if the
networking card is ifconfig down-ed? I.e. perhaps it's related to DMA
done by the networking device. Playing with BIOS settings / DMA/PCI
priorities might help reduce DMA related latencies ...

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