Hi,
I have a multi threaded userspace application running on RT_PREEMPT
linux on ARM Cortex A57 (ARMv8). The linux version is 4.14.0 with the
corresponding RT_PREEMPT patch. The below code is a simplified version
to re-create the issue. When the code is compiled with optimization
level -O1 and above, it error's out after sometime, with random value
in fl_val(result of the multiplication in thr_fn) at Line-24. After
looking at the compiled assembly code and through gdb, noticed that,
the value of 0.002 is loaded into a floating point register(s8) before
the looping starts at Line-21. The s8 register value is then used to
multiply with register storing frand value at Line-24. At random
times, the value in s8, has garbage value and the process aborts as
expected. It looks like, during kernel context switches, it is not
properly re-storing the values of floating point registers. The same
code works fine without any errors, on a non-rt kernel of the same
version on the same hardware.
My Questions
1) Is there any floating point related kernel setting that I should
set in the RT_PREEMPT kernel? I have set eagerfpu=on (even though it
is on by default)
2) Was reading about "Lazy Stacking" for Cortex-M4, but not sure if it
applies for Cortex A57
Any comments will be greatly appreciated.
----------------------------------------------Test-code------------------------------------------------------------
1 #include <iostream>
2 #include <cstdlib>
3 #include <cstdint>
4 #include <ctime>
5 #include <random>
6 #include <pthread.h>
7 #include <string.h>
8 #include <errno.h>
9 #include <unistd.h>
10 #include <signal.h>
11
12 #define MAX_THR 30
13
14 typedef float float32_t;
15 int32_t gflag = 0;
16
17 void *thr_fn(void *arg) {
18 //pthread_t cur_tid = pthread_self();
19 std::random_device rd; // non-deterministic generator
20 std::mt19937 gen(rd());
21 while (1) {
22 uint32_t rand_num = gen() % 1000;
23 float32_t frand = static_cast<float32_t>(rand_num);
24 float32_t fl_val = (frand * 0.002F);
25 if ((fl_val < 0.0F) || (fl_val > 2.0F)) {
26 gflag = -1;
27 // Keep sleeping till the process gets aborted by the main thread
28 while(1){usleep(2000);}
29 }
30 usleep(1000);
31 }
32 }
33
34 int main() {
35 int thr_cnt;
36 int res = 0;
37 pthread_t tid;
38 for (thr_cnt = 0; thr_cnt < MAX_THR; thr_cnt++) {
39 res = pthread_create(&tid, NULL, thr_fn, NULL);
40 if (res != 0) {
41 std::cout <<"Error creating pthread: " <<strerror(errno);
42 }
43 }
44 while(1){
45 if (gflag != 0) {
46 kill(getpid(), SIGABRT);
47 }
48 usleep(1000 * 1000 * 1);
49 }
50 return (0);
51 }
~
Thanks
Anup
