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Subjectrealtime scheduling problems with 2.4 linux kernel >= 2.4.10
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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, &param) < 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, &param) < 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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