linux-kernel - setitimer works incorrectly

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Message-ID: <20240913-b217598c0-81d4abf24530@bugzilla.kernel.org>
Date: Fri, 13 Sep 2024 22:00:10 +0000
From: Bugspray Bot <bugbot@...nel.org>
To: tglx@...utronix.de, bugs@...ts.linux.dev, frederic@...nel.org, 
 anna-maria@...utronix.de, linux-kernel@...r.kernel.org
Subject: setitimer works incorrectly

vladimir.mezentsev writes via Kernel.org Bugzilla:

A small test below shows the problem with the itimer setting on OL8 (x86_64 / aarch64)

% cat sig.c
#include<stdio.h>
#include <stdlib.h>
#include<signal.h>
#include <time.h>
#include <sys/time.h>
#include <string.h>

#ifndef MY_TIMER
#define MY_TIMER 1000
#endif

static time_t start;
static int sigprof_cnt = 0;

time_t
gethrtime (void)
{
  struct timespec tp;
  time_t rc = 0;
  int r = clock_gettime (CLOCK_MONOTONIC, &tp);
  if (r == 0)
    rc = ((time_t) tp.tv_sec) * 1e9 + (time_t) tp.tv_nsec;
  return rc;
}

static void sigprof_handler(int signo, siginfo_t* info, void *context)
{
  if (++sigprof_cnt >= 3)
    exit(0);
  static struct itimerval t;
  memset(&t, 0, sizeof(t));
  if (getitimer(ITIMER_PROF, &t) != 0)
    printf("getitimer failed\n");
  printf("sigprof_handler: it_interval.tv_sec=%lld it_interval.tv_usec=%lld\n"
         "                    it_value.tv_sec=%lld it_value.tv_usec=%lld\n",
    (long long) t.it_interval.tv_sec, (long long) t.it_interval.tv_usec,
    (long long) t.it_value.tv_sec, (long long) t.it_value.tv_usec);
}

volatile long x; /* temp variable for long calculation */

int
main (int argc, char **argv)
{
  long long count = 0;
  start = gethrtime ();

  struct sigaction sa;
  memset(&sa, 0, sizeof(struct sigaction));
  sa.sa_sigaction = sigprof_handler;
  sa.sa_flags = SA_RESTART | SA_SIGINFO;
  sigemptyset(&sa.sa_mask);

  if (sigaction(SIGPROF, &sa, NULL) == -1)
    {
       perror("sigaction");
       return 1;
    }

  static struct itimerval timer;
  memset(&timer, 0, sizeof(timer));
  timer.it_interval.tv_usec = MY_TIMER;
  timer.it_value.tv_usec = MY_TIMER;
  if (setitimer(ITIMER_PROF, &timer, NULL) != 0)
    {
      printf("Timer could not be initialized \n");
      return 1;
    }

  struct itimerval t;
  memset(&t, 0, sizeof(t));
  if (getitimer(ITIMER_PROF, &t) != 0)
    {
      printf("getitimer failed\n");
      return 1;
    }
  printf("After setitimer: it_interval.tv_sec=%lld it_interval.tv_usec=%lld\n"
         "                    it_value.tv_sec=%lld it_value.tv_usec=%lld\n",
    (long long) t.it_interval.tv_sec, (long long) t.it_interval.tv_usec,
    (long long) t.it_value.tv_sec, (long long) t.it_value.tv_usec);

  do
    {
      x = 0;
      for (int j = 0; j < 1000000; j++)
        x = x + 1;
      count++;
    }
  while (start + 1e9 / 4 > gethrtime ());
  printf("count=%lld  x=%lld\n", count, x);
  return 0;
}



It is from man page:
% man setitimer
...
   setitimer()
       If either field in new_value.it_value is nonzero, then the timer is arme to initially expire at the specified time.


But this is not right on x86_64 and aarch64. I run this test on OL8.

On x86_64 / OL8:
% gcc -DMY_TIMER=1000 sig.c; ./a.out
After setitimer: it_interval.tv_sec=0   it_interval.tv_usec=1000
                    it_value.tv_sec=0      it_value.tv_usec=2000 <<<<<< this should be <= 1000 because I set it_value.tv_usec to 1000 in setitimer

sigprof_handler: it_interval.tv_sec=0   it_interval.tv_usec=1000
                    it_value.tv_sec=0      it_value.tv_usec=23
sigprof_handler: it_interval.tv_sec=0   it_interval.tv_usec=1000
                    it_value.tv_sec=0      it_value.tv_usec=27



On aarch64 / OL8:

% gcc -DMY_TIMER=1000 sig.c; ./a.out
After setitimer: it_interval.tv_sec=0   it_interval.tv_usec=1000
                    it_value.tv_sec=0 it_value.tv_usec=5000    <<<<<< Same as on x86_64

sigprof_handler: it_interval.tv_sec=0   it_interval.tv_usec=1000
                    it_value.tv_sec=0 it_value.tv_usec=4000 <<<<<< this must be <= 1000 because it_interval.tv_usec is 1000

sigprof_handler: it_interval.tv_sec=0   it_interval.tv_usec=1000
                    it_value.tv_sec=0      it_value.tv_usec=4000<<<<<< this must be <= 1000 because it_interval.tv_usec is 1000

View: https://bugzilla.kernel.org/show_bug.cgi?id=217598#c0
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