Re: [patch] voluntary-preempt-2.6.9-rc1-bk4-R1

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

Ingo Molnar wrote:
> * Mark_H_Johnson@raytheon.com <Mark_H_Johnson@raytheon.com> wrote:
> 
> 
>>If you look at the date / time of the traces, you will notice that
>>most occur in the latter part of the test. This is during the "disk
>>copy" and "disk read" parts of the testing. [...]
> 
> 
> would it be possible to test with DMA disabled? (hdparm -d0 /dev/hda) It
> might take some extra work to shun the extra latency reports from the
> PIO IDE path (which is quite slow) but once that is done you should be
> able to see whether these long 0.5 msec delays remain even if all (most)
> DMA activity has been eliminated.
> 
> 
>>preemption latency trace v1.0.5 on 2.6.9-rc1-VP-R1
>>--------------------------------------------------
>> latency: 550 us, entries: 6 (6)
>>    -----------------
>>    | task: cat/6771, uid:0 nice:0 policy:0 rt_prio:0
>>    -----------------
>> => started at: kmap_atomic+0x23/0xe0
>> => ended at:   kunmap_atomic+0x7b/0xa0
>>=======>
>>00000001 0.000ms (+0.000ms): kmap_atomic (file_read_actor)
>>00000001 0.000ms (+0.000ms): page_address (file_read_actor)
>>00000001 0.000ms (+0.549ms): __copy_to_user_ll (file_read_actor)
>>00000001 0.550ms (+0.000ms): kunmap_atomic (file_read_actor)
>>00000001 0.550ms (+0.000ms): sub_preempt_count (kunmap_atomic)
>>00000001 0.550ms (+0.000ms): update_max_trace (check_preempt_timing)
>
> 
> this is a full page copy, from userspace into a kernelspace pagecache
> page. This shouldnt take 500 usecs on any hardware. Since this is a
> single instruction (memcpy's rep; movsl instruction) there's nothing
> that Linux can do to avoid (or even to cause) such a situation.

I saw this one (or one very similar) on a system that I just started 
testing on some today. Not quite as high (~219 usec if I remember 
correctly). I don't have access to the system from here. I will forward 
the trace tomorrow when I'm there tomorrow. However, I haven't seen this 
on my slower system running the same stress tests.  There are several 
possible points of interest:

System I saw this on:
P4 2.4GHz or 3.0GHz
2GB memory
2.6.9-rc1-bk12-S0 built for SMP (even though hyperthreading is off 
currently)

System I haven't seen this on:
PII 450
256MB memory
2.6.9-rc1-bk12-R6 built for UP

Sorry I don't have more complete data in front of me. I will send the 
concrete info tomorrow with the trace.

kr