Re: [patch] voluntary-preempt-2.6.8.1-P2

[Thomas Charbonnel <thomas@undata.org>]

Ingo Molnar wrote :
> here's -P2:
> 
>  http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.8.1-P2
> 
> Changes since -P1:
> 
>  - trace interrupted kernel code (via hardirqs, NMIs and pagefaults)
> 
>  - yet another shot at trying to fix the IO-APIC/USB issues.
> 
>  - mcount speedups - tracing should be faster
> 
> 	Ingo

I think I stumbled across some bugs/false positives.
Those tests were run with acpi=off, so that this specific issue doesn't
interfere. voluntary_preemption is set to 3 throughout.

The first problem was already reported by Lee Revell. Creating a new tab
in gnome-terminal gives :

[...]
 0.064ms (+0.000ms): preempt_schedule (try_to_wake_up)
 0.065ms (+0.000ms): preempt_schedule (copy_page_range)
 0.065ms (+0.000ms): preempt_schedule (copy_page_range)
[... plenty of preempt_schedule (copy_page_range) skipped ...]
 0.202ms (+0.000ms): preempt_schedule (copy_page_range)
 0.202ms (+0.000ms): preempt_schedule (copy_page_range)
 0.202ms (+0.000ms): check_preempt_timing (touch_preempt_timing)

This is induced by kernel_preemption=0 and disappears with
kernel_preemption=1. It seems to be a side-effect of the current design.
Is there a way to avoid this ?

The second one still involves creating a new tab in gnome-terminal, but
with kernel_preemption=1 :

preemption latency trace v1.0
-----------------------------
 latency: 130 us, entries: 6 (6)
 process: gnome-terminal/14381, uid: 1000
 nice: 0, policy: 0, rt_priority: 0
=======>
 0.000ms (+0.000ms): __vma_link_rb (copy_mm)
 0.000ms (+0.000ms): rb_insert_color (copy_mm)
 0.000ms (+0.000ms): __rb_rotate_left (rb_insert_color)
 0.000ms (+0.000ms): copy_page_range (copy_mm)
 0.000ms (+0.000ms): pte_alloc_map (copy_page_range)
 0.127ms (+0.126ms): check_preempt_timing (touch_preempt_timing)

When entering check_preempt_timing, preempt_thresh was 0, and
preempt_max_latency had been freshly reset to 100. It should have
triggered this code :
		if (latency < preempt_max_latency)
			goto out;
but for some reason it didn't (or there is a problem in the tracing
code, not showing events that would have increased 'latency').

Thomas