Hi Dietmar, Attention !, these tests were executed on a kernel with the patch RT and the option CONFIG_PREEMPT_RT_FULL = y. I confirm the truth of my priority settings On a vanilla kernel, I get the same results as you.
After talking with mike Galbraith, I turned my attention to the priority of kernel threads. The following link explains in the behavior of the scheduler : https://wiki.linuxfoundation.org/realtime/documentation/technical_details/hr_timers Contrary to what I thought, there is no dynamic adjustment of the priority according to the priority of the calling task. -----Message d'origine----- De : linux-kernel-ow...@vger.kernel.org [mailto:linux-kernel-ow...@vger.kernel.org] De la part de Dietmar Eggemann Envoyé : mercredi 6 février 2019 11:55 À : Frédéric Mathieu <frederic.math...@dualis.com>; linux-kernel@vger.kernel.org Objet : Re: Kernel 4.9: strange behavior with fifo scheduler Hi Frédéric, On 2/5/19 11:47 AM, Frédéric Mathieu wrote: > Hi, > > on an X86_64 architecture (Intel(R) Core(TM) i3-6100U CPU @ 2.30GHz), > I use the linux kernel 4.9.146 with patch rt 125. > uname -a: Linux 4.9.146-rt125 #1 SMP PREEMPT RT Tue Jan 29 14:17:55 > CET 2019 > x86_64 GNU/Linux > > I observed a strange behavior of the scheduler when several tasks are > executed in FIFO mode on a CPU core and a significant CPU activity. > > first test (reference: cpu load=0%): > cyclictest -m -D 5 -i 1000 -p 50 -a 0 > # / dev / cpu_dma_latency set to 0us > policy: fifo: loadavg: 1.95 1.06 0.43 1/159 14305 > T: 0 (14145) P: 50 I: 1000 C: 4997 Min: 7 Act: 7 Avg: 7 Max: > 18 work fine > > now, i'm loading the system on the cpu core 0 with a homemade process: > cpu load 60%, sched FIFO prio 1, cpu 0 Are you sure that your test app runs with prio 1? Is this in the range of the SCHED_FIFO (userspace) priorities shown by chrt -m? ... SCHED_FIFO min/max priority : 1/99 ... If I run your setup (test and cyclictest affine to CPU0) on 4.15.0-43 (i7-4750HQ) with: (1) test prio > cyclictest prio # chrt -p $PID_TEST pid 28489's current scheduling policy: SCHED_FIFO pid 28489's current scheduling priority: 51 # cat /proc/$PID_TEST/stat 28489 (test) R 28488 28487 8664 34828 28487 4194304 86 0 0 0 0 0 0 0 -52 I get your behaviour: # /dev/cpu_dma_latency set to 0us Thread 0 Interval: 1500 0: 0: 6 0: 1: 5 0: 2: 2 0: 3: 5419 0: 4: 3 0: 5: 2 0: 6: 2 0: 7: 2 0: 8: 5422 0: 9: 3 ... whereas with: (2) test prio < cyclictest prio # chrt -p $PID_TEST pid 28811's current scheduling policy: SCHED_FIFO pid 28811's current scheduling priority: 49 # cat /proc/$PID_TEST/stat 28811 (test) S 28810 28809 8664 34828 28809 1077936128 90 0 0 0 0 0 0 0 -50 I get: # /dev/cpu_dma_latency set to 0us Thread 0 Interval: 1500 0: 0: 7 0: 1: 4 0: 2: 3 0: 3: 5 0: 4: 4 0: 5: 2 0: 6: 2 0: 7: 2 0: 8: 2 0: 9: 3 ... [...] > In this case cyclictest detects very long latencies > cyclictest -m -D 5 -i 1000 -p 50 -a 0 -v > > Max CPUs = 2 > # /dev/cpu_dma_latency set to 0us > Thread 0 Interval: 1500 > 0: 0: 13 > 0: 1: 8 > 0: 2: 7 > 0: 3: 5648 > 0: 4: 8 > 0: 5: 7 > 0: 6: 7 > 0: 7: 7 > 0: 8: 5649 [...] > After verification, although no other process is running with real > time scheduler, I see a latency of about 5.6 ms at regular intervals. > > it seems that the priority task 1 (fifo) is not pre-empted by the > cyclictest process with a priority of 50 (fifo) when the low priority task is > active. > This corresponds to the cycle recorded in the file: 6 ms of latency > followed by 4 "normal" latencies of 7 us. > > Does anyone have any idea of this problem? > > Best regards > Frederic MATHIEU
