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

Esben Nielsen
2005-10-14T21:36:07Z

On Fri, 14 Oct 2005, Ingo Molnar wrote:

>
> * 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?
I have been testing on this box before but never with such long latencies
before.
> Occasionally there are boxes
> that show clear signs of hardware latencies - there's little the kernel
> can do about those.
I actually felt like the box went to sleep.

>
> Wild shot in the dark: are there any power-saving modes enabled on the
> box?

I have had problems with power-saving before on this box. The effect is
usually the other way around: The rtdcs used to meassure the timing slows
down. I.e. it ought to meassure very short times, not very long times.

I'll try to disable power-management.

> 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 ...

Hmm. If it is a DMA transfer for the network device it ought to make bad
latencies for any network traffic not only NFS. I will have to test
various things during the weekend.

Esben

>
> Ingo
> -
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