[ELDK] Problem with pthread_cond_wait on 8xx?

[Detlev Zundel]

Hi Frank,

> On Wed, Sep 23, 2009 at 2:13 PM, Detlev Zundel <dzu at denx.de> wrote:
>> Hi Frank,
>>
>>> I have a problem with high CPU load on 8xx when using pthread_cond_wait.
>>> I'm using the glibc v2.6 that came with ELDK 4.2, compiled with NPTL
>>> support. The same code compiled for x86 works fine (but then using
>>> glibc 2.9). I'm running torvalds mainline kernel v2.6.26-rc2.
>>>
>>> Can anyone here using ELDK 4.2 on 8xx or another PowerPC target try to
>>> compile the program listed below and check the CPU load when running
>>> it? Or pinpoint a problem with the code.
>>>
>>> My current assumption is that the glibc pthread_cond_wait call is
>>> implemented using
>>> busywaiting/spinlocking on 8xx, or something is wrong in the kernel.
>>
>> Actually I can reproduce both of your behaviours on my x86 and on a
>> PowerPC system ;)
>>
>> I don't understand why this is the case, but if I compile the program
>> without "-lpthread" I have ~100% CPU utilization on both systems.  If I
>> do "the right thing" and compile with "-lpthread" everything is fine on
>> both systems.
>>
>
> Thanks for testing the code ;-)
>
> What PPC target did you test it on, what version of the standard C library
> do you use, and what kernel?

I used a MPC5200B system for testing with our ELDK 4.2 toolchain on a
2.6.29.2 kernel.  Out of lazyness, I compiled the test program natively.

> If I don't link it against libpthread, I get link errors (unresolved
> functions). I assume you're using the same toolchain (ELDK 4.2), so
> this is strange. IOW, I have to link with -lpthread, and you
> don't. How come?

How should I know? ;) This was really an error on my side forgetting to
specify "-lpthread" on the first shot (on x86 gcc 4.3.3-8 glibc 2.9
linux 2.6.30.1) and I was kind of perplexed to see no link errors.  So I
tried the same on native ELDK 4.2 and got the same results.

So let me retest with the cross-compiler:

[dzu at pollux dzu]$ eldk-switch tqm5200
[ tqm5200 is using MPC5200 ]
Setup for ppc_6xx (using ELDK 4.2)
Adjusted /home/dzu/target-root pointing to /opt/eldk-4.2/ppc_6xx
[dzu at pollux dzu]$ ${CROSS_COMPILE}gcc -o cond_wait_test cond_wait_test.c
[dzu at pollux dzu]$ 

Works....

>> Maybe someone here on this list can explain this?
>
> Yes I really hope so. The best would be that someone could pinpoint
> a problem with the example code, and why the behaviour is different on
> different targets. 

Reading the code, I see absolutely no problem.

What perplexed me however is that I could see the same thing on my x86
host.  There I even took a look with ltrace and see that without
"-lpthread" there is really a continous stream of library calls, whereas
with "-lpthread", it is one single blocking call...

> In case I don't find any solution, I will probably need to
> upgrade/downgrade the kernel, switch to using uclibc/eglibc, or upgrade
> glibc to v2.9. Do you plan to upgrade the ELDK anytime soon (ie. newer
> glibc for instance)?

We do not currently have a project which would finance such an upgrade,
I'm afraid.

> FYI: I've used getconf and verified that I'm using the NPTL enabled pthreads
> library on the 8xx.

In your position, I'd try to reproduce my findings on the x86 host and
get an explanation there first why two completely different behaviours
occur.  When you have understood why this happens, maybe you can verify
this for the PowerPC environment.

Cheers
  Detlev

-- 
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