2.5 kernel regression in alarm() syscall behaviour?

Begin forwarded message:

Date: Mon, 14 Jul 2003 09:39:33 -0500
From: Amos Waterland <[email protected]>
To: [email protected]
Subject: Re: 2.5 kernel regression in alarm() syscall behaviour?

I think Wes' mail client mangled his testcase a bit. Here is a cleaned up version.

Compile with:

% gcc -Wall -Werror alarm.c -o alarm

Output on 2.4 kernel is:

% ./alarm
[1058193354] alarm(0), want retval:0; got retval:0 (PASS)
...
[1058193354] alarm(9), want retval:8; got retval:8 (PASS) 0/10 tests failed

Output on 2.5 kernel is: many failures. The number of failures go down when the system is heavily stressed.

---- Begin alarm.c ----

#include <unistd.h>
#include <stdio.h>
#include <sys/time.h>

#define MINVAL 0
#define MAXVAL 10
#define NOALARM 0

int main(int argc, char **argv)

{
    int retval = 0, failed = 0, tests = MAXVAL, prev = 0, curr = 0;
    struct timeval time;

    if (argc > 1)
        if (sscanf(argv[1], "%d", &tests) != 1)
            return 1;

    for (curr = MINVAL; curr < tests; curr++) {
        retval = alarm(curr);
        gettimeofday(&time, NULL);
        printf("[%li] alarm(%d), want retval:%d; ",
               time.tv_sec, curr, prev);
        /* was there a previous alarm? */
        if (retval == NOALARM && prev == NOALARM) {
            printf("got retval:0 (PASS)");
        } else if (retval == NOALARM && prev > NOALARM) {
            printf("got retval:0 (FAIL)");
            failed++;
        } else if (retval != prev) {
            printf("got retval:%d (FAIL)", retval);
            failed++;
        } else {
            printf("got retval:%d (PASS)", retval);
        }
        printf("\n");
        prev = curr;
    }
    printf("%d/%d tests failed\n", failed, tests);
    return failed;
}

---- End alarm.c ----
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I suppose we are going to have a lot of these. The test calls alarm which sets up an itimer for the specified number of seconds and returns the number of seconds remaining on the old itimer. If any useconds remain, seconds is boosted by 1. The test expects the number returned to be the same as what was sent, i.e. 1 second wait is expected to return 1 second if it is immeadiatly queried.

The problem with this test is that it assumes that seconds can be translated into jiffies with out any error. Jiffies, however, is now defined to be close but not equal to 1/HZ. In fact, on the x86 jiffies is 999848 nano seconds. The conversion of a second with the proper round up gives 1001 jiffies and converting this back to seconds gives 1.000847848 seconds. It is this 0.000847848 that is forcing the subject test to report a number higher than expected.

IMHO it is the test that is wrong, not the kernel.

Quoted 15 lines · george anzinger

I suppose we are going to have a lot of these. The test calls alarm which sets up an itimer for the specified number of seconds and returns the number of seconds remaining on the old itimer. If any useconds remain, seconds is boosted by 1. The test expects the number returned to be the same as what was sent, i.e. 1 second wait is expected to return 1 second if it is immeadiatly queried.

The problem with this test is that it assumes that seconds can be translated into jiffies with out any error. Jiffies, however, is now defined to be close but not equal to 1/HZ. In fact, on the x86 jiffies is 999848 nano seconds. The conversion of a second with the proper round up gives 1001 jiffies and converting this back to seconds gives 1.000847848 seconds. It is this 0.000847848 that is forcing the subject test to report a number higher than expected.

Quoted 1 line

IMHO it is the test that is wrong, not the kernel.

Thanks for your explanation.

The language of the SuSv3[1] seems a little vague:

If there is a previous alarm() request with time remaining, alarm() shall return a non-zero value that is the number of seconds until the previous request would have generated a SIGALRM signal.

It leaves open to interpretation the question of what the return value should be if the delta between calling alarm(1) and alarm(n) is near 0s, about .5s, and near 1s. I think the issue can be boiled down to an implementation choice of floor(delta), round(delta), ceil(delta).

Linux 2.4, Solaris, OpenBSD take the floor() approach. Running alarm2.c[2] on them gives results similar to the following; where 'delta' is the difference in microseconds between calling alarm(1) and alarm(2), and 'ret' is the return value of the alarm(2) call:

t1      t2      delta   ret
------  ------  ------  ------
940284  947913  7629    1
948336  57230   108894  1
57697   266358  208661  1
266826  576979  310153  1
577455  986953  409498  1
987447  497109  509662  1
497576  106690  609114  1
107173  817304  710131  1
817767  626956  809189  1

Alarm clock

That is, even when 809ms have passed between calling alarm(1) and alarm(2), Linux 2.4/Solaris/OpenBSD report that there is 1s "until the previous request would have generated a SIGALRM signal."

Since 2.5 is incrementing tv_sec if any tv_usec remain, it is much closer to taking a ceil() approach: 1.000847848 gets converted to 2, rather than 1.

I guess the question boils down to: does Linux 2.6 want to make a different implementation choice than 2.4 and other Unix variants?

----

[1] http://www.opengroup.org/onlinepubs/007904975/functions/alarm.html

[2]

#include <unistd.h>
#include <stdio.h>
#include <sys/time.h>

/* Returns the difference in usecs between now and before.
*/
long delta_t(struct timeval now, struct timeval before)

{
    if (now.tv_sec == before.tv_sec && now.tv_usec >= before.tv_usec) {
        return now.tv_usec - before.tv_usec;
    } else if (now.tv_sec > before.tv_sec) {
        return (now.tv_usec + 1000000) - before.tv_usec;
    }

    return -1;
}

int main(int argc, char **argv)

{
    int i, ret, fails = 0;
    struct timeval t1, t2;

    printf("t1\tt2\tdelta\tret\n------\t------\t------\t------\n");

    for (i = 0; ; i += 100000) {
        gettimeofday(&t1, NULL);

        alarm(1);
        usleep(i);
        ret = alarm(2);

        gettimeofday(&t2, NULL);

        printf("%li\t%li\t%li\t%i\n", t1.tv_usec,
               t2.tv_usec, delta_t(t2, t1), ret);
    }

    return fails;
}