On Tuesday 22 May 2018 06:20:25 Lampersperger Andreas wrote: > Hello Jordan, > > IMHO this is because the non-RT Tasks uses the same CPU and therefore > the same cache. Especially when your CPU has hyperthreading also the > L1-Cache is shared between RT and non-RT. Try using different CPUs for > RT and non-RT tasks und disable hyperthreading (in the bios). > > @all: When I'm not correct, I would be very pleased if somebody let me > know about this. > > Best regards > Andreas > > -----Ursprüngliche Nachricht----- > Von: linux-rt-users-owner@xxxxxxxxxxxxxxx > [mailto:linux-rt-users-owner@xxxxxxxxxxxxxxx] Im Auftrag von Jordan > Palacios Gesendet: Dienstag, 22. Mai 2018 12:00 > An: linux-rt-users@xxxxxxxxxxxxxxx > Betreff: Non RT threads impact on RT thread > > Hello, > > We are currently running a version of the linux kernel (3.18.24) with > the RT-PREEMPT patch. In our system there are several non RT tasks and > one RT task. The RT process runs in the FIFO scheduler with 95 > priority and a control loop of 1ms. > > We have achieved latencies of about 5us which are perfect for us. > > Our issue is that the RT task sometimes misses one of its cycles due > to an unexpected very long execution time of its control loop. In our > system this is a critical failure. > There is also the kernel argument isolcpus, where in a multicore machine, it makes the cpu # passed disappear from the schedulers control, and linuxcnc then runs its main realtime stuff on this isolated cpu core. And that can be a real help with this. > We enabled tracing in the kernel and started measuring the execution > time of the RT thread. The execution time is quite constant (about > 200us), which random spikes every now and then. Thing is, the less non > RT tasks running in the system the better the RT task behaves. > > We wrote a very simple RT application that does some light work and > writes its execution time using the trace_marker. Execution time is > constant but IO intensive stuff, like a stress --io 32 or a hdparm, > will have and impact on its execution time. This is surprising because > the test does not any kind of work related to IO. Nor does the RT task > in our system for that matter. > > Our question is: Is this behaviour normal? Why are non RT tasks > affecting the RT task performance? Is there any other kind of test > that we could run that would shed some light on this issue? > > Thanks in advance. > > Jordan. > > > > #include <limits.h> > #include <pthread.h> > #include <sched.h> > #include <stdio.h> > #include <stdlib.h> > #include <sys/mman.h> > #include <sys/types.h> > #include <sys/syscall.h> > #include <sys/stat.h> > #include <fcntl.h> > #include <sstream> > #include <math.h> > #include <unistd.h> > #include <stdarg.h> > > typedef long long NANO_TIME; > typedef struct timespec TIME_SPEC; > > static const long int INTERVAL = 1000000LL; // 1ms > > static TIME_SPEC nano2timespec(NANO_TIME hrt) { > TIME_SPEC timevl; > timevl.tv_sec = hrt / 1000000000LL; > timevl.tv_nsec = hrt % 1000000000LL; > return timevl; > } > > static double to_us(NANO_TIME ns) > { > return static_cast<double>(ns) / 1000.0; } > > static NANO_TIME get_time_ns(void) > { > TIME_SPEC tv; > clock_gettime(CLOCK_REALTIME, &tv); > return NANO_TIME(tv.tv_sec) * 1000000000LL + NANO_TIME(tv.tv_nsec); > } > > static const unsigned int WORK_SIZE = 10000; > > void do_work() > { > unsigned int i = 0; > double aux = 2; > > // do some stuff to use cpu > while (i < WORK_SIZE) > { > aux = pow(aux, i); > ++i; > } > } > > int trace_fd = -1; > > void trace_write(const char* fmt, ...) > { > va_list ap; > char buf[256]; > int n; > > if (trace_fd < 0) > return; > > va_start(ap, fmt); > n = vsnprintf(buf, 256, fmt, ap); > va_end(ap); > > write(trace_fd, buf, n); > } > > void* thread_func(void* /*data*/) > { > pid_t tid = syscall(__NR_gettid); > printf("pthread joined. TID %i \n", tid); > > TIME_SPEC ts_next; > NANO_TIME wake, init, after_work; > > wake = get_time_ns() + INTERVAL; > ts_next = nano2timespec(wake); > > while (true) > { > init = get_time_ns(); > > if (init < wake) > { > do_work(); > after_work = get_time_ns(); > > const double exec_time = to_us(after_work - init); > trace_write("rt_test_exec_time - %f", exec_time); > } > else > { > printf("overrun detected\n"); > } > > clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &(ts_next), NULL); > wake = wake + INTERVAL; > ts_next = nano2timespec(wake); > } > return NULL; > } > > int main(int /*argc*/, char* /*argv*/[]) { > struct sched_param param; > pthread_attr_t attr; > pthread_t thread; > int ret; > > /* Lock memory */ > if (mlockall(MCL_CURRENT | MCL_FUTURE) == -1) > { > printf("mlockall failed: %m\n"); > exit(-2); > } > > /* Allocate all the memory required by the process */ > if (!malloc(100 * 1048576)) // 100MB > { > printf("malloc failed\n"); > exit(-3); > } > > /* Initialize pthread attributes (default values) */ > ret = pthread_attr_init(&attr); > if (ret) > { > printf("init pthread attributes failed\n"); > return ret; > } > > /* Set a specific stack size */ > ret = pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN * 100); > if (ret) > { > printf("pthread setstacksize failed\n"); > return ret; > } > > /* Set scheduler policy and priority of pthread */ > ret = pthread_attr_setschedpolicy(&attr, SCHED_FIFO); > if (ret) > { > printf("pthread setschedpolicy failed\n"); > return ret; > } > > param.sched_priority = 95; > ret = pthread_attr_setschedparam(&attr, ¶m); > if (ret) > { > printf("pthread setschedparam failed\n"); > return ret; > } > > /* Use scheduling parameters of attr */ > ret = pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED); > if (ret) > { > printf("pthread setinheritsched failed\n"); > return ret; > } > > /* Create a pthread with specified attributes */ > ret = pthread_create(&thread, &attr, thread_func, NULL); > if (ret) > { > printf("create pthread failed\n"); > return ret; > } > > /* Open tracer_marker */ > trace_fd = open("/tracing/trace_marker", O_WRONLY); > if (trace_fd == -1) > { > printf("trace_marker open failed\n"); > return ret; > } > > system("echo 1 > /tracing/tracing_on"); // enable > tracing system("echo nop > /tracing/current_tracer"); // reset > trace system("echo function > /tracing/current_tracer"); // use > function tracer system("echo 1 > /tracing/events/enable"); // > enable all events > > /* Join the thread and wait until it is done */ > ret = pthread_join(thread, NULL); > if (ret) > printf("join pthread failed: %m\n"); > > system("echo 0 > /tracing/tracing_on"); // disable tracing > system("echo 0 > /tracing/events/enable"); // disable all events > > return ret; > } > -- > To unsubscribe from this list: send the line "unsubscribe > linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > > ---------------------------------------------------------------------- >-------------------------------- Registergericht: Traunstein / Registry > Court: HRB 275 - Sitz / Head Office: Traunreut > Aufsichtsratsvorsitzender / Chairman of Supervisory Board: Rainer > Burkhard Geschäftsführung / Management Board: Thomas Sesselmann > (Vorsitzender / Chairman), Hubert Ermer, Michael Grimm, Lutz Rissing > > E-Mail Haftungsausschluss / E-Mail Disclaimer: > http://www.heidenhain.de/disclaimer > N������y��X��ǧv��){.n�����{��۬z�"��>^n�r������&����(�階�ݢj"���� >��ޖ�����h���~�m� -- Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. 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