Fwd: Help with debugging a POSIX timing test.

Chirayu Desai chirayudesai1 at gmail.com
Wed Nov 27 09:30:57 UTC 2013


On 27 November 2013 02:45, Joel Sherrill <joel.sherrill at oarcorp.com> wrote:

> Hi
>
> This should be closer than the version you posted
> on Melange. I added printf's to help show where
> the context switches from one thread to another
> were occurring.  See if you can follow what is
> going on.
>
Yes, I'm able to understand the context switches now.

>
> Remember the test description is:
>
> pthread_setschedparam - lower own priority, preempt

which means that the benchmark operation starts
> in one thread which lowers its priority allowing
> another thread to run. When that thread runs it
> will stop the timer and report.
>
I missed that, my bad.

>
> Feel free to ask questions about what the code
> is doing.
>
The printfs made it really clear this time, thanks.

>
> On 11/25/2013 9:49 PM, Chirayu Desai wrote:
> >
> >
> >
> > On 26 November 2013 00:40, Joel Sherrill <joel.sherrill at oarcorp.com
> > <mailto:joel.sherrill at oarcorp.com>> wrote:
> >
> >     On 11/25/2013 10:58 AM, Chirayu Desai wrote:
> >     >
> >     > Sorry, I sent the last message to only Joel.
> >
> >     I MIGHT have eventually gotten to it. But am swamped.
> >
> >     > ---------- Forwarded message ----------
> >     > From: *Chirayu Desai* <chirayudesai1 at gmail.com
> >     <mailto:chirayudesai1 at gmail.com>
> >     > <mailto:chirayudesai1 at gmail.com <mailto:chirayudesai1 at gmail.com>>>
> >     > Date: 25 November 2013 15:30
> >     > Subject: Re: Help with debugging a POSIX timing test.
> >     > To: Joel Sherrill <joel.sherrill at oarcorp.com
> >     <mailto:joel.sherrill at oarcorp.com>
> >     > <mailto:joel.sherrill at oarcorp.com <mailto:
> joel.sherrill at oarcorp.com>>>
> >     >
> >     >
> >     >
> >     >
> >     >
> >     > On 25 November 2013 00:35, Joel Sherrill
> >     <joel.sherrill at oarcorp.com <mailto:joel.sherrill at oarcorp.com>
> >     > <mailto:joel.sherrill at oarcorp.com
> >     <mailto:joel.sherrill at oarcorp.com>>> wrote:
> >     >
> >     >     Now that I can see and run the code, a few things
> >     >     jump out.
> >     >
> >     >     + POSIX priorities -- lower numerically ==> more important
> >     >     + You did &Thread_Id to calls after created. The & isn't
> >     >     supposed to be there.
> >     >
> >     >     + &policy should be the second argument to
> >     >     pthread_getschedparam.
> >     >
> >     > This explains the ESRCH I was getting.
> >
> >     Yep. And one of the compiler warnings as well. :)
> >
> >     >     + Pay attention to compiler warnings. :)
> >     >
> >     > Sorry for not doing so.
> >
> >     No problem. This is a good example of how properly addressing
> >     the warning would have fixed the issue with no time in the
> >     debugger. Good programming practices try to keep you out of
> >     a debugger. :)
> >
> >     >     + Benchmark time is initialized IMMEDIATELY BEFORE the
> >     >     single operation under test. We try to avoid including
> >     >     anything.
> >     >
> >     > Got it.
> >     >
> >     >
> >     >     I have attached a new version of init.c with comments
> >     >     hacked in and changes.
> >     >
> >     > Thanks.
> >     >
> >     >
> >     >     The big thing I tried to put in a comment block is that
> >     >     the way this test is structured, it includes the hidden
> >     >     start up time for the first time test_thread(0 runs.
> >     >     I tried to write up notes on how to modify the test
> >     >     to avoid that.
> >     >
> >     > I was unable to understand all of it.
> >     > From what I understood, POSIX_Init is called first, which
> >     > cals benchmark_pthread_setschedparam.
> >     > That creates a new thread, gets the priority and policy, and then
> >     > setschedparam is called
> >     > with the new (lowered) priority, which is what we want to test.
> >
> >     pthread_create() creates and starts the thread. If it is
> >     more important than POSIX_Init(), it will immediately be
> >     switched to and run. But it isn't so it won't run until
> >     POSIX_Init() lowers its priority.
> >
> >
> > That makes it much more clear.
> >
> >
> >
> >     >     For convenience, I would add a helper routine like
> >     >     this:
> >     >
> >     >     void set_thread_priority( id, new_priority )
> >     >
> >     >     and call it. It will greatly simplify the code.
> >     >
> >     > Noted, I will do that after I get a better understanding of the
> code.
> >
> >     Since you will be changing priority multiple times to switch
> >     back and forth, this will really help tighten the code.
> >
> > Done [0] :)
> >
> >
> >
> >     >     I hope I didn't fall into the inverse
> >     >     priority range trap in those instructions....
> >     >
> >     >     WARNING: POSIX priorities run INVERSE from the internal
> >     >     priorities but in gdb if you print:
> >     >
> >     >     p _Per_CPU_Information.per_cpu.executing->current_priority
> >     >
> >     >     You will see the internal priority (NOT POSIX priority)
> >     >     of the currently running thread. 1 is most important
> >     >     and 255 is the IDLE task.
> >     >
> >     > I'm confused.
> >     > Per
> >
> http://www.rtems.org/onlinedocs/doxygen/cpukit/html/group__POSIX__PRIORITY.html#gada0c9a015d42fd545af7454f1ca0d098
> ,
> >     > "RTEMS Core has priorities run in the opposite sense of the POSIX
> >     API."
> >     > So, for this task, lowering the POSIX priority is what we want, it
> is
> >     > the output I'm getting which confuses me
> >     >
> >     > Original priority: 2
> >     > Lowered priority: 4
> >
> >     I am going to do this as a mix of internal and
> >     POSIX priorities
> >
> >     Internal 255 is the lowest priority and illegal in POSIX.
> >     Internal 253 = POSIX_Init() at start (POSIX 2)
> >        see cpukit/posix/src/pthread.c for the default attributes
> >
> >     Where you say lowered, it is actually becoming more important
> >     and moving to a numeric value with numbers above and below.
> >     At 2, there is little room below it. It is the next to lowest
> >     POSIX priority value.
> >
> >     test_thread() is created at priority 2 also because the
> >     attributes are NULL.
> >
> >     What you print as "Lowered priority" is actually the priority
> >     of test_thread() if I am reading things correctly.
> >
> >     Hint: Make your set priority helper take (const char *, id, priority)
> >     and you can print the thread name in debug messages. :)
> >
> >     When I break at test_thread(), the priority is POSIX=4, Internal=251
> >
> > I'm getting the hang of this now.
> >
> >
> >     >     So the numbers you pick are important to switch back and
> >     >     forth between the tasks.
> >     >
> >     >     I think the test is pretty close in spite of all that I
> >     >     wrong. I stepped through the code attached and it is
> >     >     doing the right thing EXCEPT including the thread hidden
> >     >     start time. :)
> >     >
> >     >     Benchmark programs are hard to get right but fun to write.
> >     >
> >     > Indeed
> >
> >     :)
> >
> >     Hope this helps.
> >
> >     Stepping in gdb and printing the priority of the current thread
> >     helps. It will be the internal priority though.
> >
> >     b POSIX_Init
> >     b test_thread
> >
> >     and use
> >
> >     p _Per_CPU_Information.per_cpu.executing->current_priority
> >
> > Using breakpoints did help, thanks.
> >
> > I have attached a new patch, and cross-posted it to melange as well.
> >
> >
> >     >
> >     >     --joel
> >     >
> >     >     On 11/24/2013 11:50 AM, Joel Sherrill wrote:
> >     >     > Sorry to be lazy/stupid but how to I download just
> >     >     > the diff to see what's going on? I am not that
> >     >     > github literate.
> >     >     >
> >     >     > --joel
> >     >     >
> >     >     > On 11/24/2013 11:28 AM, Chirayu Desai wrote:
> >     >     >> Hello everyone.
> >     >     >>
> >     >     >> I am Chirayu Desai, a high school student, currently
> >     participating in
> >     >     >> Google Code-In 2013
> >     >     >>
> >     >     >> I have currently working on the task [0], but I'm having
> >     some trouble
> >     >     >> trying to get my code[1] to work.
> >     >     >>
> >     >     >> The task is to create a POSIX timing test psxtmthread05.
> >     >     >> The test case is: pthread_setschedparam() - lower own
> priority.
> >     >     >> I managed to write up something [2], but it doesn't work.
> >     >     >> The GDB output is:
> >     >     >>
> >     >     >> (gdb) r
> >     >     >> Starting program:
> >     >     >>
> >     >
> >
> /home/cdesai/rtems/b-sis/sparc-rtems4.11/c/sis/testsuites/psxtmtests/psxtmthread05/psxtmthread05.exe
> >     >     >>
> >     >     >>
> >     >     >> *** POSIX TIME TEST PSXTMTHREAD05 ***
> >     >     >> getschedparam: 3
> >     >     >> Original priority: 5
> >     >     >> Lowered priority: 4
> >     >     >> setschedparam: 3
> >     >     >> pthread_setschedparam - lower own priority 2226
> >     >     >> *** END OF POSIX TIME TEST PSXTMTHREAD05 ***
> >     >     >> [Inferior 1 (process 42000) exited normally]
> >     >     >>
> >     >     >> [0]:
> >     >     >>
> >     >
> >
> http://www.google-melange.com/gci/task/view/google/gci2013/6383096106582016
> >     >     >> [1]:
> https://github.com/chirayudesai/rtems/tree/psxtmthread05
> >     >     >> [2]:
> >     >
> >
> https://github.com/chirayudesai/rtems/commit/890cebf084ca2a3815e3049a766276ddcdb0188a
> >     >     >>
> >     >     >> P.S. This is my first post to this list, so excuse me for
> any
> >     >     mistakes.
> >     >     >>
> >     >     >> Regards,
> >     >     >> Chirayu Desai
> >     >     >
> >     >     >
> >     >     >
> >     >
> >     >
> >     >
> >     >     --
> >     >     Joel Sherrill, Ph.D.             Director of Research &
> >     Development
> >     >     joel.sherrill at OARcorp.com        On-Line Applications Research
> >     >     Ask me about RTEMS: a free RTOS  Huntsville AL 35805
> >     >     Support Available                (256) 722-9985
> >     >
> >     >
> >     >
> >
> >
> >
> >     --
> >     Joel Sherrill, Ph.D.             Director of Research & Development
> >     joel.sherrill at OARcorp.com        On-Line Applications Research
> >     Ask me about RTEMS: a free RTOS  Huntsville AL 35805
> >     Support Available                (256) 722-9985
> >
> >
>
>
>
> --
> Joel Sherrill, Ph.D.             Director of Research & Development
> joel.sherrill at OARcorp.com        On-Line Applications Research
> Ask me about RTEMS: a free RTOS  Huntsville AL 35805
> Support Available                (256) 722-9985
>
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