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Date: Tue, 5 Feb 2019 11:47:30 +0100
From: Frédéric Mathieu <frederic.mathieu@...lis.com>
To: <linux-kernel@...r.kernel.org>
Subject: Kernel 4.9: strange behavior with fifo scheduler
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
code snippet
active_time = 6000;
sleep_time = 4000;
do {
clock_gettime(CLOCK_MONOTONIC, &start_time);
// active loop
do {
for( cptdelay=0; cptdelay <
100; cptdelay++ );
clock_gettime(CLOCK_MONOTONIC, &now);
diff = timespec_diff_us(
&start_time, &now );
} while (diff < active_time);
// sleep : suspend time
usleep(sleep_time);
} while( 1 );
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
0: 9: 7
0: 10: 7
0: 11: 7
0: 12: 7
0: 13: 5651
0: 14: 7
0: 15: 7
0: 16: 7
0: 17: 7
0: 18: 5655
0: 19: 7
0: 20: 7
0: 21: 7
0: 22: 7
0: 23: 5658
0: 24: 7
0: 25: 7
0: 26: 7
0: 27: 7
0: 28: 5663
0: 29: 7
0: 30: 7
0: 31: 7
0: 32: 7
0: 33: 5664
0: 34: 7
0: 35: 7
0: 36: 7
0: 37: 7
0: 38: 5667
0: 39: 7
0: 40: 7
0: 41: 7
0: 42: 7
0: 43: 5671
0: 44: 8
0: 45: 7
0: 46: 7
0: 47: 7
0: 48: 5673
0: 49: 7
0: 50: 7
0: 51: 7
0: 52: 7
0: 53: 5676
0: 54: 7
0: 55: 7
0: 56: 7
0: 57: 7
0: 58: 5679
0: 59: 7
0: 60: 7
0: 61: 7
0: 62: 7
0: 63: 5682
0: 64: 7
0: 65: 7
0: 66: 7
0: 67: 7
0: 68: 5685
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
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