itimers: Fix periodic tics precision

Measure ITIMER_PROF and ITIMER_VIRT timers interval error
between real ticks and requested by user. Take it into account
when scheduling next tick.

This patch introduce possibility where time between two
consecutive tics is smaller then requested interval, it
preserve however dependency that n tick is generated not
earlier than n*interval time - counting from the beginning of
periodic signal generation.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
LKML-Reference: <1248862529-6063-3-git-send-email-sgruszka@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3b3efaddd9536272714d75cffc01d934a44b4be5..a069e65e8bb7f00c64de15226ccc0056dce1ab0a 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -473,6 +473,8 @@ struct pacct_struct {
 struct cpu_itimer {
        cputime_t expires;
        cputime_t incr;
+       u32 error;
+       u32 incr_error;
 };
 
 /**

diff --git a/kernel/itimer.c b/kernel/itimer.c
index 852c88ddd1f027189076f61b84ce43cbc2774f88..21adff7b2a177548f161baff978d119e65ac429c 100644 (file)
--- a/kernel/itimer.c
+++ b/kernel/itimer.c
@@ -42,7 +42,7 @@ static struct timeval itimer_get_remtime(struct hrtimer *timer)
 }
 
 static void get_cpu_itimer(struct task_struct *tsk, unsigned int clock_id,
-                          struct itimerval *value)
+                          struct itimerval *const value)
 {
        cputime_t cval, cinterval;
        struct cpu_itimer *it = &tsk->signal->it[clock_id];
@@ -127,14 +127,32 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
        return HRTIMER_NORESTART;
 }
 
+static inline u32 cputime_sub_ns(cputime_t ct, s64 real_ns)
+{
+       struct timespec ts;
+       s64 cpu_ns;
+
+       cputime_to_timespec(ct, &ts);
+       cpu_ns = timespec_to_ns(&ts);
+
+       return (cpu_ns <= real_ns) ? 0 : cpu_ns - real_ns;
+}
+
 static void set_cpu_itimer(struct task_struct *tsk, unsigned int clock_id,
-                          struct itimerval *value, struct itimerval *ovalue)
+                          const struct itimerval *const value,
+                          struct itimerval *const ovalue)
 {
-       cputime_t cval, cinterval, nval, ninterval;
+       cputime_t cval, nval, cinterval, ninterval;
+       s64 ns_ninterval, ns_nval;
        struct cpu_itimer *it = &tsk->signal->it[clock_id];
 
        nval = timeval_to_cputime(&value->it_value);
+       ns_nval = timeval_to_ns(&value->it_value);
        ninterval = timeval_to_cputime(&value->it_interval);
+       ns_ninterval = timeval_to_ns(&value->it_interval);
+
+       it->incr_error = cputime_sub_ns(ninterval, ns_ninterval);
+       it->error = cputime_sub_ns(nval, ns_nval);
 
        spin_lock_irq(&tsk->sighand->siglock);
 

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9b2d5e4dc8c4a5f61ca8cbb285e15487f901dacb..b60d644ea4b307f267b7a0146869b54bcd1d8404 100644 (file)
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1070,6 +1070,8 @@ static void stop_process_timers(struct task_struct *tsk)
        spin_unlock_irqrestore(&cputimer->lock, flags);
 }
 
+static u32 onecputick;
+
 static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
                             cputime_t *expires, cputime_t cur_time, int signo)
 {
@@ -1077,9 +1079,16 @@ static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
                return;
 
        if (cputime_ge(cur_time, it->expires)) {
-               it->expires = it->incr;
-               if (!cputime_eq(it->expires, cputime_zero))
-                       it->expires = cputime_add(it->expires, cur_time);
+               if (!cputime_eq(it->incr, cputime_zero)) {
+                       it->expires = cputime_add(it->expires, it->incr);
+                       it->error += it->incr_error;
+                       if (it->error >= onecputick) {
+                               it->expires = cputime_sub(it->expires,
+                                                       jiffies_to_cputime(1));
+                               it->error -= onecputick;
+                       }
+               } else
+                       it->expires = cputime_zero;
 
                __group_send_sig_info(signo, SEND_SIG_PRIV, tsk);
        }
@@ -1696,10 +1705,15 @@ static __init int init_posix_cpu_timers(void)
                .nsleep = thread_cpu_nsleep,
                .nsleep_restart = thread_cpu_nsleep_restart,
        };
+       struct timespec ts;
 
        register_posix_clock(CLOCK_PROCESS_CPUTIME_ID, &process);
        register_posix_clock(CLOCK_THREAD_CPUTIME_ID, &thread);
 
+       cputime_to_timespec(jiffies_to_cputime(1), &ts);
+       onecputick = ts.tv_nsec;
+       WARN_ON(ts.tv_sec != 0);
+
        return 0;
 }
 __initcall(init_posix_cpu_timers);