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

[K.R. Foley]

K.R. Foley wrote:
> Ingo Molnar wrote:
>
> > * Mark_H_Johnson@xxxxxxxxxxxx <Mark_H_Johnson@xxxxxxxxxxxx> 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)

Actual system info for above is 2.4GHz with 512 memory.

> 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.

As promised here is the trace:

http://www.cybsft.com/testresults/2.6.9-rc1-bk12-S0/trace2.txt

I also have some other traces from this system that I have not seen 
before on my slower system. For instance this one where we spend ~204 
usec in __spin_lock_irqsave:

http://www.cybsft.com/testresults/2.6.9-rc1-bk12-S0/trace1.txt

Or this one where we spend ~203 usec in sched_clock. That just doesn't 
seem possible.

http://www.cybsft.com/testresults/2.6.9-rc1-bk12-S0/trace4.txt

kr
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