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

[Ingo Molnar <mingo@elte.hu>]

* Esben Nielsen <simlo@phys.au.dk> 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 ...

	Ingo