Hi, I have been experimenting with the FIFO scheduler and tick rate in order to reduce execution speed for tasks; I need to run legacy code that runs on old slow hardware at approximately the same speed on new faster hardware while preserving "realtime-like" properties of the code. The general idea was to add a delay during each task, tick to mimic the execution speed of the slower system. At the end of this mail are my modifications in the kernel, where I for the FIFO scheduler introduces a delay of 500 microseconds for each scheduling tick, that is my intention is to use up about 500 microseconds of the scheduling tick for the task. Assuming a tick length of 1000 microseconds we effectively reduce the execution speed to about 1 - 500/1000 or about 50%. I have made a small test program to verify that my approach works as expected which I run in userspace (delay_test.c): #include <stdio.h> #include <unistd.h> #include <time.h> #include <sched.h> #include <stdint.h> #include <sys/time.h> static inline void timespec_diff(struct timespec *a, struct timespec *b, struct timespec *result) { result->tv_sec = a->tv_sec - b->tv_sec; result->tv_nsec = a->tv_nsec - b->tv_nsec; if (result->tv_nsec < 0) { --result->tv_sec; result->tv_nsec += 1000000000L; } } void main(int argc, char *args[]) { struct sched_param param; param.sched_priority = 99; sched_setscheduler(0,SCHED_FIFO,¶m); struct timespec start,now,diff; uint64_t sleep_delay = 100L; clock_gettime(CLOCK_MONOTONIC, &start); printf("Start is %lu.%lu\n",start.tv_sec,start.tv_nsec); for(;;) { usleep(sleep_delay); clock_gettime(CLOCK_MONOTONIC, &now); timespec_diff(&now,&start,&diff); printf("Time is %lus %luns\n",(uint64_t)diff.tv_sec,(uint64_t)diff.tv_nsec); } } I compile the above program like this (assuming a 2GHz CPU core that is evacuated and assigned a dedicated Cgroup) $ gcc .... and create it like this with Cgroup v1 sudo cgcreate -t $USER:$GRP -a $USER:$GRP -g cpuset,memory,freezer:tes echo "1" | sudo tee /sys/fs/cgroup/cpuset/test/cpuset.cpus > /dev/null echo "0" | sudo tee /sys/fs/cgroup/cpuset/test/cpuset.mems > /dev/null echo "1" | sudo tee /sys/fs/cgroup/cpuset/test/cpuset.cpu_exclusive > /dev/null and run it with sudo cgexec -g memory,freezer,cpuset:test ./a.out The expected output is something like Start is 1835.474539419 Time is 0s 200000ns Time is 0s 300000ns Time is 0s 500000ns Time is 0s 1000000ns <-- Here scheduler delay kicks i. Time is 0s 1100000ns Time is 0s 1200000ns ... and so on. Instead, I get this Start is 1835.474539419 Time is 0s 175382ns Time is 0s 283956ns Time is 0s 391289ns Time is 0s 501217ns <-- Around here scheduler delay should have kicked in. Time is 0s 610443ns <- Instead roughly the same delay is added. Time is 0s 721963ns Time is 0s 835387ns Time is 0s 948290ns Time is 0s 1061414ns Things I have tried so far: Turning off SMT, making sure that I am using CONFIG_HZ_PERIODIC , Isolating tasks using cgroups and isolcpus, making sure that the system is running at the correct tick rate and changing the frequency in combination with the Governor. Is it something I am missing in regards to doing this slowdown? Any answers or hints would be appreciated. My own delay method I have added to the kernel (register_time.c): static uint64_t rdtsc2(void){ uint32_t hi, lo; __asm__ __volatile__ ("xorl %%eax,%%eax\ncpuid" ::: "%rax", "%rbx", "%rcx", "%rdx"); __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi)); return (((uint64_t)hi)<<32) | ((uint64_t)lo) ; } uint64_t get_khz(void){ uint64_t aperf,mperf,khz; unsigned long flags; local_irq_save(flags); rdmsrl(MSR_IA32_APERF,aperf); rdmsrl(MSR_IA32_MPERF,mperf); local_irq_restore(flags); khz = div64_u64((cpu_khz * aperf),mperf); return khz; } uint64_t nanos_to_ticks(uint64_t nanos){ return (get_khz() * 1000L * nanos)/NSEC_PER_SEC; } /* *Sleeps for the specified amount of ticks. * Ticks to sleep can be calculated with nanos_to_tick(). * Going from ticks to nanoseconds is done using ticks_to_nanos(). */ uint64_t tick_sleep(uint64_t ticks_to_sleep) { uint64_t now; uint64_t sleep_until; uint64_t delta_ticks; preempt_disable(); now = rdtsc2(); sleep_until = now + ticks_to_sleep; while (now < sleep_until) { now = rdtsc2(); } delta_ticks = now - sleep_until; preempt_enable(); return delta_ticks; } The part where I try to add the delay (rt.c): static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued) { struct sched_rt_entity *rt_se = &p->rt; update_curr_rt(rq); update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 1); watchdog(rq, p); /* * RR tasks need a special form of timeslice management. * FIFO tasks have no timeslices. * CONFIG_PWM_FRAMEWORK is used to activate the special FIFO */ #ifdef CONFIG_PWM_FRAMEWORK if (p->policy == SCHED_FIFO) { tick_sleep(nanos_to_ticks(500000)); return; } #else if (p->policy != SCHED_RR) { return; } #endif if (--p->rt.time_slice) return; p->rt.time_slice = sched_rr_timeslice; /* * Requeue to the end of the queue if we (and all of our ancestors) are not * the only element on the queue */ for_each_sched_rt_entity(rt_se) { if (rt_se->run_list.prev != rt_se->run_list.next) { requeue_task_rt(rq, p, 0); resched_curr(rq); return; } } } Best Regards Jacob M