Messages in this thread | | | From | Ian Collinson <> | Subject | realtime scheduling problems with 2.4 linux kernel >= 2.4.10 | Date | Thu, 30 May 2002 18:54:46 +0100 |
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We're having problems with realtime scheduling (SCHED_RR and SCHED_FIFO), on 2.4 kernels >= 2.4.10 (built for i386, no SMP). We have an app that uses real-time scheduled threads. To aid debugging, in case of realtime threads spinning and locking the system, we always keep a bash running on a (text) console, at SCHED_RR, priority 99 (a higher priority than any threads in our app). We test that this is a valid approach by running a lower priority realtime app, on another console, that sits in an infinite busy loop. This has always worked, and we've been able to successfully use the high-priority bash to run gdb, and so on. This is also what the man page for sched_setscheduler suggests, to avoid total system lock up.
But, it seems to have stopped working with 2.4 kernels >= 2.4.10, (have tried up to 2.4.19-pre9). Once the lower priority realtime process is running, the higher priority shell doesn't pre-empt it, and locks up.
Details of the test:
I have a "realtime" app, that sets the scheduling policy and priority, then forks a specified app, and "eat_cpu", that just sits in an infinite loop. Both are attached.
On one console (as root), I do
> realtime -rr 99 bash Which gives me a priorty 99, SCHED_RR bash. Then, on another console (also as root), I do:
> sleep 10; realtime -rr 50 eat_cpu
Then I switch back to the first console, with its priority 99 bash. I am able to type away for 10 seconds, until the priority 50 process starts, at which point the shell locks up. I can get the same effect on one console with:
> ( sleep 10; realtime -rr 50 eat_cpu ) & realtime -rr 99 bash
Previously, the high-priority shell would never lock up. Now it does.
Any idea why this doesn't work?
Cheers, Ian
Code:
/* realtime * To build: * g++ -o realtime realtime.cxx */
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <sched.h> #include <unistd.h> #include <string>
void usage() { printf("usage: realtime -get | {-rr | -fifo} <pri> cmd ...\n"); exit(1); }
const char *policyString(int policy) { switch(policy) { case SCHED_OTHER: return "SCHED_OTHER"; case SCHED_FIFO: return "SCHED_FIFO"; case SCHED_RR: return "SCHED_RR"; } return "BAD POLICY"; }
int main(int argc, char *argv[]) { seteuid(getuid());
if (argc < 2) usage();
if (strcmp(argv[1], "-get") == 0) { int policy = sched_getscheduler(0); if (policy < 0) { perror("sched_getscheduler failed:"); return 1; }
sched_param param; if (sched_getparam(0, ¶m) < 0) { perror("sched_getparam failed"); return 1; }
printf("policy = %d = %s\n", policy, policyString(policy));
if (policy != SCHED_OTHER) printf("priority=%d\n", param.sched_priority);
int max = sched_get_priority_max(policy); if (max < 0) { perror("sched_get_priority_max"); return 1; } int min = sched_get_priority_min(policy); if (min < 0) { perror("sched_get_priority_min"); return 1; } printf("prorities: min = %d, max = %d\n", min, max);
return 0;
}
// set
if (argc < 4) usage();
int policy;
if (!strcmp(argv[1], "-rr")) { policy = SCHED_RR; } else if (!strcmp(argv[1], "-fifo")) { policy = SCHED_FIFO; } else { printf("bad policy: choose -rr (round robin) or -fifo\n"); usage(); } struct sched_param param; param.sched_priority = atoi(argv[2]);
printf("policy = %d = %s\n", policy, policyString(policy)); printf("priority=%d\n", param.sched_priority);
// become root seteuid(0);
if (sched_setscheduler(0, policy, ¶m) < 0) { perror("sched_setscheduler"); return 1; }
// back down seteuid(getuid());
string cmd; for (int i=3; i<argc; i++) cmd = cmd + argv[i] + " ";
printf("cmd = %s\n", cmd.c_str());
system(cmd.c_str());
return 0; }
/* eof */
/* eat_cpu.c */ int main(int argc, char *argv[]) { for (;;) ; } /* eof */
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