Found with 'stress-ng --cpu-sched' from current stress-ng upstream HEAD:
Testcase (attached):
$ gcc -O2 -o manysignals manysignals.c
$ ./manysignals
fork() = 1833
...
fork() = 1848
...
kill(1833, 17)
...
kill(1848, 17)
kill(1833, 9)
...
kill(1848, 9)
waitpid(1833, ., 0)
Run this in second terminal:
$ watch "ps | sed -n '1p;/manysignals/{/sed/d;p}'"
If 'S' appear in the first column, the child processes likely reached
the final SIGSTOP state. This takes some time. The parent process may
still hang in first waitpid() but should not.
If the parent process is aborted with ^C, child processes may be stopped
or left behind. Occasionally a child process that can not be stopped by
Cygwin (kill -9) is left behind.
Tested with ancient (i7-2600K) and more recent (i7-14700K) CPU :-)
Unrelated to the above, but related to 'stress-ng --cpu-sched' which
uses sched_get/setscheduler():
- sched_getscheduler() always returns SCHED_FIFO. As far as I understand
Linux sched(7), this is a non-preemptive real-time policy. The
preemptive SCHED_RR would possibly a more reasonable value.
Unfortunately SCHED_OTHER cannot be used because it would require to
ignore the priority.
- sched_setscheduler() always fails with ENOSYS. It IMO should allow to
set 'param->sched_priority' if 'policy' is equal to the value returned
by sched_getscheduler().
--
Regards,
Christian
#define _GNU_SOURCE
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <wait.h>
static void xkill(pid_t pid, int sig)
{
printf("kill(%d, %d)\n", (int)pid, sig);
int ret = kill(pid, sig);
if (ret)
perror("kill");
}
int main()
{
// number of child processes
const int nprocs = 16;
// number of SIGSTOP+SIGCONT, ..., SIGSTOP+SIGCONT, SIGSTOP.
const int nstopcont = 10;
pid_t pids[nprocs];
for (int p = 0; p < nprocs; p++) {
pid_t pid = fork();
if (pid == (pid_t)-1) {
perror("fork"); return 1;
}
if (pid == 0) {
cpu_set_t cpus; CPU_ZERO(&cpus);
CPU_SET(0, &cpus);
if (sched_setaffinity(getpid(), sizeof(cpus), &cpus))
perror("setaffinity");
for (;;)
sched_yield();
}
printf("fork() = %d\n", (int)pid);
pids[p] = pid;
}
sleep(1);
for (int i = 0; ; ) {
for (int p = 0; p < nprocs; p++)
xkill(pids[p], SIGSTOP);
if (++i >= nstopcont)
break;
for (int p = 0; p < nprocs; p++)
xkill(pids[p], SIGCONT);
}
for (int p = 0; p < nprocs; p++)
xkill(pids[p], SIGKILL);
for (int p = 0; p < nprocs; p++) {
pid_t pid = pids[p];
printf("waitpid(%d, ., 0)\n", (int)pid); fflush(stdout);
int status;
pid_t ret = waitpid(pid, &status, 0);
if (ret == (pid_t)-1)
perror("waitpid");
}
return 0;
}
--
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