#include <stdio.h> // printf, puts, perror
#include <string.h> // memset
#include <unistd.h> // sleep
#include <signal.h> // sig*
#include <sched.h> // sched_setscheduler
#include <time.h> // clock_gettime, timer_*
#define ZERO(type, name) type name; memset(&name, 0, sizeof name)
#define SIG_TIMER_EXPIRATION SIGRTMIN
#define CLOCK_TYPE CLOCK_MONOTONIC
void set_SCHED_RR(void)
{
ZERO(struct sched_param, param);
param.sched_priority = 66;
if (sched_setscheduler(0, SCHED_RR, ¶m) != 0) perror("sched_setscheduler");
}
struct timespec get_time_stamp(void)
{
ZERO(struct timespec, now);
if (clock_gettime(CLOCK_TYPE, &now) != 0) perror("clock_gettime");
printf("NOW=%ld.%09ld\n", now.tv_sec, now.tv_nsec);
return now;
}
int main(void)
{
set_SCHED_RR();
sigset_t set;
sigemptyset(&set);
sigaddset(&set, SIG_TIMER_EXPIRATION);
sigprocmask(SIG_BLOCK, &set, NULL);
ZERO(struct timespec, reso);
if (clock_getres(CLOCK_TYPE, &reso) != 0) perror("clock_getres");
printf("RESOLUTION=%ld ns\n", reso.tv_sec*1000000000+reso.tv_nsec);
timer_t timer;
ZERO(struct sigevent, event);
event.sigev_notify = SIGEV_SIGNAL;
event.sigev_signo = SIG_TIMER_EXPIRATION;
if (timer_create(CLOCK_TYPE, &event, &timer) != 0) perror("timer_create");
sigset_t pending;
struct timespec H = get_time_stamp();
puts("SLEEPING 1 SECOND..."); sleep(1);
/* H is now 1 second in the past. */
get_time_stamp();
ZERO(struct itimerspec, spec);
spec.it_value = H;
if (timer_settime(timer, TIMER_ABSTIME, &spec, NULL) != 0) perror("timer_settime");
if (sigpending(&pending) != 0) perror("sigpending");
int res=sigismember(&pending, SIG_TIMER_EXPIRATION);
if (res < 0) perror("sigismember");
else if (res) puts("TIMER HAS ALREADY FIRED.");
struct timespec H1 = get_time_stamp();
sigwaitinfo(&set, NULL);
struct timespec H2 = get_time_stamp();
int nsdiff = (H2.tv_sec-H1.tv_sec)*1000000000 + (H2.tv_nsec-H1.tv_nsec);
printf("nsdiff=%d ns i.e. %d ms\n", nsdiff, nsdiff/1000000);
return 0;
}
John wrote:
> I'm playing with the POSIX timers API. My platform is x86 running Linux
> 2.6.18.1 patched with the high-resolution timer subsystem.
>
> http://www.tglx.de/hrtimers.html
>
> I'm seeing unexpected behavior from timer_settime().
>
> int timer_settime(timer_t timerid, int flags,
> const struct itimerspec *value, struct itimerspec *ovalue);
>
> timer_settime() is used to arm a timer. If the TIMER_ABSTIME flag is
> set, then the timer should fire when the appropriate clock reaches the
> date specified by value. If that date is in the past, the timer should
> fire immediately.
>
> The Open Group Base Specifications Issue 6 states:
> http://www.opengroup.org/onlinepubs/009695399/functions/timer_getoverrun.html
>
>
> "If the flag TIMER_ABSTIME is set in the argument flags, timer_settime()
> shall behave as if the time until next expiration is set to be equal to
> the difference between the absolute time specified by the it_value
> member of value and the current value of the clock associated with
> timerid. That is, the timer shall expire when the clock reaches the
> value specified by the it_value member of value. If the specified time
> has already passed, the function shall succeed and the expiration
> notification shall be made."
>
> In my tests, when timer_settime() is called with an expiration date in
> the past, the timer still takes some time to fire.
>
> Here's a run-down of the code provided as an attachment:
>
> I switch to a SCHED_RR scheduling policy. In other words, whenever my
> process wants the CPU, it gets it. (No other SCHED_RR or SCHED_FIFO
> processes on the system.) I mask the signal that will be delivered on
> timer expiration. I then arm a timer with an expiration date in the
> past, check whether the signal is pending, and block waiting for the
> signal. I then print how long I've had to wait.
>
> # ./a.out
> RESOLUTION=1 ns
> NOW=969.735545919
> SLEEPING 1 SECOND...
> NOW=970.735581398
> NOW=970.735613525
> NOW=970.735749017
> nsdiff=135492 ns i.e. 135.5 ?s
>
> Any ideas?
Is there a better forum to discuss this matter?
Regards.
On Mon, 27 Nov 2006 15:32:21 +0100
John <[email protected]> wrote:
> John wrote:
>
> > I'm playing with the POSIX timers API. My platform is x86 running Linux
> > 2.6.18.1 patched with the high-resolution timer subsystem.
> >
> > http://www.tglx.de/hrtimers.html
> >
> > I'm seeing unexpected behavior from timer_settime().
> >
> > int timer_settime(timer_t timerid, int flags,
> > const struct itimerspec *value, struct itimerspec *ovalue);
> >
> > timer_settime() is used to arm a timer. If the TIMER_ABSTIME flag is
> > set, then the timer should fire when the appropriate clock reaches the
> > date specified by value. If that date is in the past, the timer should
> > fire immediately.
> >
> > The Open Group Base Specifications Issue 6 states:
> > http://www.opengroup.org/onlinepubs/009695399/functions/timer_getoverrun.html
> >
> >
> > "If the flag TIMER_ABSTIME is set in the argument flags, timer_settime()
> > shall behave as if the time until next expiration is set to be equal to
> > the difference between the absolute time specified by the it_value
> > member of value and the current value of the clock associated with
> > timerid. That is, the timer shall expire when the clock reaches the
> > value specified by the it_value member of value. If the specified time
> > has already passed, the function shall succeed and the expiration
> > notification shall be made."
> >
> > In my tests, when timer_settime() is called with an expiration date in
> > the past, the timer still takes some time to fire.
> >
> > Here's a run-down of the code provided as an attachment:
> >
> > I switch to a SCHED_RR scheduling policy. In other words, whenever my
> > process wants the CPU, it gets it. (No other SCHED_RR or SCHED_FIFO
> > processes on the system.) I mask the signal that will be delivered on
> > timer expiration. I then arm a timer with an expiration date in the
> > past, check whether the signal is pending, and block waiting for the
> > signal. I then print how long I've had to wait.
> >
> > # ./a.out
> > RESOLUTION=1 ns
> > NOW=969.735545919
> > SLEEPING 1 SECOND...
> > NOW=970.735581398
> > NOW=970.735613525
> > NOW=970.735749017
> > nsdiff=135492 ns i.e. 135.5 ?s
> >
> > Any ideas?
>
> Is there a better forum to discuss this matter?
>
It hasn't been forgotten about.
This problem, plus the dynticks-makes-us-disable-the-TSC problem, plus
[email protected]'s-synaptics driver are all (IMO)
blocking a merge.
On Fri, 2006-11-17 at 10:11 +0100, John wrote:
> value specified by the it_value member of value. If the specified time
> has already passed, the function shall succeed and the expiration
> notification shall be made."
That's exactly what the implementation does:
The functions succeeds (return value is 0) and then the notification is
made. There is _NO_ requirement that the signal is delivered before the
function returns to user space. I could be implemented, but I'm not sure
if it is worth the effort.
tglx