pthread_cond_timedwait() broken in 9-stable? (from JAN 10)

[David Xu]

On 2012/2/17 16:06, Julian Elischer wrote:
> On 2/16/12 11:41 PM, Julian Elischer wrote:
>> adding jkim as he seems to be the last person working with TSC.
>>
>>
>> On 2/16/12 6:42 PM, David Xu wrote:
>>> On 2012/2/17 10:19, Julian Elischer wrote:
>>>> On 2/16/12 5:56 PM, David Xu wrote:
>>>>> On 2012/2/17 8:42, Julian Elischer wrote:
>>>>>> Adding David Xu for his thoughts since he reqrote the code in 
>>>>>> quesiton in revision 213098
>>>>>>
>>>>>> On 2/16/12 2:57 PM, Julian Elischer wrote:
>>>>>>> On 2/16/12 1:06 PM, Julian Elischer wrote:
>>>>>>>> On 2/16/12 9:34 AM, Andriy Gapon wrote:
>>>>>>>>> on 15/02/2012 23:41 Julian Elischer said the following:
>>>>>>>>>> The program fio (an IO test in ports) uses pthreads
>>>>>>>>>>
>>>>>>>>>> the following code (from fio-2.0.3, but its in earlier code too)
>>>>>>>>>> has suddenly started misbehaving.
>>>>>>>>>>
>>>>>>>>>>          clock_gettime(CLOCK_REALTIME,&t);
>>>>>>>>>>          t.tv_sec += seconds + 10;
>>>>>>>>>>
>>>>>>>>>>          pthread_mutex_lock(&mutex->lock);
>>>>>>>>>>
>>>>>>>>>>          while (!mutex->value&&  !ret) {
>>>>>>>>>>                  mutex->waiters++;
>>>>>>>>>>                  ret = 
>>>>>>>>>> pthread_cond_timedwait(&mutex->cond,&mutex->lock,&t);
>>>>>>>>>>                  mutex->waiters--;
>>>>>>>>>>          }
>>>>>>>>>>
>>>>>>>>>>          if (!ret) {
>>>>>>>>>>                  mutex->value--;
>>>>>>>>>>                  pthread_mutex_unlock(&mutex->lock);
>>>>>>>>>>          }
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> It turns out that 'ret' sometimes comes back instantly (on my 
>>>>>>>>>> machine) with a
>>>>>>>>>> value of 60 (ETIMEDOUT)
>>>>>>>>>> despite the fact that we set the timeout 10 seconds into the 
>>>>>>>>>> future.
>>>>>>>>>>
>>>>>>>>>> Has anyone else seen anything like this?
>>>>>>>>>> (and yes the condition variable attribute have been set to 
>>>>>>>>>> use the REALTIME clock).
>>>>>>>>> But why?
>>>>>>>>>
>>>>>>>>> Just a hypothesis that maybe there is some issue with time 
>>>>>>>>> keeping on that system.
>>>>>>>>> How would that code work out for you with MONOTONIC?
>>>>>>>>
>>>>>>>> Jens Axboe, (CC'd) tried both CLOCK_REALTIME and 
>>>>>>>> CLOCK_MONOTONIC, and they both had the same problem..
>>>>>>>> i.e. random early returns with ETIMEDOUT.
>>>>>>>>
>>>>>>>> I think we will try move out machine forward to a newer -stable 
>>>>>>>> to see if it resolves.
>>>>>>> Kan upgraded the machine today to today's 9.x branch tip and the 
>>>>>>> problem still occurs.
>>>>>>> 8.x does not have this problem.
>>>>>>>
>>>>>>> I have not got a 9-RELEASE machine to test on.. so I can not 
>>>>>>> tell if this came in with the burst of stuff
>>>>>>> that came in after the 9.x branch was unfrozen after the release 
>>>>>>> of 9.0.
>>>>>>>
>>>>>>>
>>>>>>
>>>>> I am trying to reproduce the problem,  do you have complete sample 
>>>>> code to test ?
>>>>
>>>> I'm still looking the exact set
>>>> but on my machine (4 cpus) the program from ports sysutils/fio 
>>>> exhibits the problem when used with
>>>> kern.timecounter.hardware=TSC-low and with the following config file:
>>>>
>>>> pu05 # cat config.fio
>>>>
>>>> [global]
>>>> #clocksource=cpu
>>>> direct=1
>>>> rw=randread
>>>> bs=4096
>>>> fill_device=1
>>>> numjobs=16
>>>> iodepth=16
>>>> #ioengine=posixaio
>>>> #ioengine=psync
>>>> ioengine=psync
>>>> group_reporting
>>>> norandommap
>>>> time_based
>>>> runtime=60000
>>>> randrepeat=0
>>>>
>>>> [file1]
>>>> filename=/dev/ada0
>>>>
>>>> pu05 #
>>>> pu05 # fio config.fio
>>>> fio: this platform does not support process shared mutexes, forcing 
>>>> use of threads. Use the 'thread' option to get rid of this warning.
>>>> file1: (g=0): rw=randread, bs=4K-4K/4K-4K, ioengine=psync, iodepth=16
>>>> ...
>>>> file1: (g=0): rw=randread, bs=4K-4K/4K-4K, ioengine=psync, iodepth=16
>>>> fio 2.0.3
>>>> Starting 15 threads and 1 process
>>>> fio: job startup hung? exiting.
>>>> fio: 5 jobs failed to start
>>>> Segmentation fault (core dumped)
>>>> pu05#
>>>>
>>>>
>>>> The reason 5 jobs failed to start is because the parent timed out 
>>>> on them immediately.
>>>> It didn't time out on 10 of them apparently.
>>>>
>>>>
>>>> if I set the timer to ACPI-fast it works as expected..
>>> maybe following code can check to see if TSC-LOW works by let the 
>>> thread run
>>> on each cpu.
>>>
>>> gettimeofday(&prev, NULL);
>>> int cpu = 0;
>>> for (;;) {
>>>      cpuset_t set;
>>>      cpu = ++cpu % 4;
>>>      CPU_ZERO(&set);
>>>      CPU_SET(cpu, &set);
>>>      pthread_setaffinity_np(pthread_self(), sizeof(set), &set);
>>>      gettimeofday(&cur, NULL);
>>>      if ( timercmp(&prev, &cur, >=)) {
>>>         abort();
>>>    }
>>> }
>>>
>>>
>
> pu05# sysctl kern.timecounter.hardware=TSC-low
> kern.timecounter.hardware: ACPI-fast -> TSC-low
> pu05# ./test
> ^C
> pu05# cat test.c
>
> #include <stdlib.h>
> #include <sys/param.h>
> #include <sys/cpuset.h>
> #include <pthread_np.h>
>
> #include <sys/time.h>
>
> main()
> {
>     int cpu = 0;
>     struct timeval prev, cur;
>
>     gettimeofday(&prev, NULL);
>     for (;;) {
>          cpuset_t set;
>          cpu = ++cpu % 4;
>          CPU_ZERO(&set);
>          CPU_SET(cpu, &set);
>          pthread_setaffinity_np(pthread_self(), sizeof(set), &set);
>          gettimeofday(&cur, NULL);
>          if ( timercmp(&prev, &cur, >)) {
>             abort();
>        }
>        prev = cur;
>     }
> }
>
> pu05# ./test
>
> minutes pass.......
>
> ^C
> pu05#
>
> so it looks as if the TSC is working ok..
> I'm just going to check that the program is actually moving CPU...
> yes it is moving around but I can't tell at what speed. (according to 
> top).
>
> so we are still left with a question of "where is the problem?"
>
> kernel TSC driver?
> generic gettimeofday() code?
> pthreads cond code?
> the application?
>
>
I am running the fio test on my notebook which is using TSC-low,
it is on 9.0-RC3, I can not reproduce the problem for
minutes, then I interrupt it with ctrl-c:

http://people.freebsd.org/~davidxu/tsc_pthread/dmesg.txt
http://people.freebsd.org/~davidxu/tsc_pthread/tc.txt
http://people.freebsd.org/~davidxu/tsc_pthread/fio.txt