Andrew Morton wrote:
>
> 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.
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.
-g
>
> 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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>
--
George Anzinger [email protected]
High-res-timers: http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml
On Mon, Jul 14, 2003 at 05:16:06PM -0700, george anzinger wrote:
> 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.
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;
}
Amos Waterland wrote:
> On Mon, Jul 14, 2003 at 05:16:06PM -0700, george anzinger wrote:
>
>>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.
>
>
> 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?
It is the glibc "alarm()" code that is doing this, not the kernel.
AND, gosh, I was hoping the standard would give some guidance here,
but it completely ignores the issue.
-g
>
> ----
>
> [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;
> }
>
>
--
George Anzinger [email protected]
High-res-timers: http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml