Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm1-V0.7.27-3

["K.R. Foley" <kr@cybsft.com>]

Florian Schmidt wrote:
> On Tue, 16 Nov 2004 22:24:01 +0100
> Ingo Molnar <mingo@elte.hu> wrote:
> 
> 
>>great. The current release is meanwhile at -V0.7.27-10, which includes
>>other minor updates:
>>
> 
> 
> Ok, this one boots fine again for me (didn't test the ones betwen my last
> report and this one).
> 
> I have not yet tried to get this kernel to lock up yet, but i made another
> interesting observation:
> 
> irq 8 at prio 98 (only irq 1 with higher prio 99). running rtc_wakeup in the
> console (it runs SCHED_FIFO allright). Switching consoles (different text
> consoles - not swithcing to X, though this basically produces similar
> results) produces large jitters (around 1 ms) and occasional missed irq's
> and piggy messages. This is completely reproducable here. The rtc histogram
> doesn't show any large wakeup latencies.

Just a thought. What priority are you running rtc_wakup at? If you are 
doing something like:

schp.sched_priority = sched_get_priority_max(SCHED_FIFO); // which 
equates to a priority of 99

Then you it is actually running at a higher priority than the rtc, and 
it won't work very well. I tend to run rtc (IRQ 8) at 99 and the 
programs accessing it at 98 which seems to work reasonably well.

> 
> /proc/latency_trace doesn't show that high latencies either on console
> switch:
> 
> preemption latency trace v1.0.7 on 2.6.10-rc2-mm1-RT-V0.7.27-10
> -------------------------------------------------------
>  latency: 63 us, entries: 22 (22)   |   [VP:0 KP:1 SP:1 HP:1 #CPUS:1]
>     -----------------
>     | task: IRQ 8/13, uid:0 nice:-5 policy:1 rt_prio:98
>     -----------------
>  => started at: try_to_wake_up+0x51/0x170 <c010f3a1>
>  => ended at:   finish_task_switch+0x51/0xb0 <c010f911>
> =======>
>     5 80010004 0.000ms (+0.000ms): trace_start_sched_wakeup (try_to_wake_up)
>     5 80010003 0.000ms (+0.000ms): (1) ((98))
>     5 80010003 0.000ms (+0.000ms): (13) ((5))
>     5 80010003 0.000ms (+0.000ms): try_to_wake_up (wake_up_process)
>     5 80010003 0.000ms (+0.000ms): (0) ((1))
>     5 80010002 0.000ms (+0.000ms): preempt_schedule (try_to_wake_up)
>     5 80010002 0.000ms (+0.000ms): wake_up_process (redirect_hardirq)
>     5 80010001 0.000ms (+0.000ms): preempt_schedule (__do_IRQ)
>     5 80010001 0.000ms (+0.000ms): irq_exit (do_IRQ)
>     5 80000002 0.000ms (+0.000ms): do_softirq (irq_exit)
>     5 80000002 0.001ms (+0.061ms): __do_softirq (do_softirq)
>     5 00000000 0.062ms (+0.000ms): preempt_schedule (_mmx_memcpy)
>     5 90000000 0.062ms (+0.000ms): __schedule (preempt_schedule)
>     5 90000000 0.062ms (+0.000ms): profile_hit (__schedule)
>     5 90000001 0.062ms (+0.000ms): sched_clock (__schedule)
>    13 80000002 0.062ms (+0.000ms): __switch_to (__schedule)
>    13 80000002 0.062ms (+0.000ms): (5) ((13))
>    13 80000002 0.062ms (+0.000ms): (98) ((1))
>    13 80000002 0.062ms (+0.000ms): finish_task_switch (__schedule)
>    13 80000001 0.062ms (+0.000ms): trace_stop_sched_switched (finish_task_switch)
>    13 80000001 0.063ms (+0.003ms): (13) ((1))
>    13 80000001 0.066ms (+0.000ms): trace_stop_sched_switched (finish_task_switch)
> 
> I sometimes do get large values in /proc/latency_trace, but they seem to be
> unrelated to the console switching.
> 
> flo
>