[PATCH resend 2/2] itimers: fix periodic tics precision

[Stanislaw Gruszka <sgruszka@redhat.com>]

Measure interval of tics generated by ITIMER_VIRT and ITIMER_PROF using ktime
instead of cputime. Calculate error between requested interval and current one,
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>
---
 include/linux/sched.h     |    3 ++-
 kernel/fork.c             |    6 ++++--
 kernel/itimer.c           |   18 ++++++++++--------
 kernel/posix-cpu-timers.c |   24 +++++++++++++++++++++---
 4 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 58430e7..e897a4a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -456,7 +456,8 @@ struct pacct_struct {
 
 struct cpu_itimer {
 	cputime_t expires;
-	cputime_t incr;
+	ktime_t	incr;
+	u32 err_ns;
 };
 
 #define IT_PROF 0
diff --git a/kernel/fork.c b/kernel/fork.c
index a6225d3..885ca45 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -791,9 +791,11 @@ static void posix_cpu_timers_init_group(struct signal_struct *sig)
 
 	/* Expiration times and increments. */
 	sig->it[IT_PROF].expires = cputime_zero;
-	sig->it[IT_PROF].incr = cputime_zero;
+	sig->it[IT_PROF].incr = ktime_set(0, 0);
+	sig->it[IT_PROF].err_ns = 0;
 	sig->it[IT_VIRT].expires = cputime_zero;
-	sig->it[IT_VIRT].incr = cputime_zero;
+	sig->it[IT_VIRT].incr = ktime_set(0, 0);
+	sig->it[IT_VIRT].err_ns = 0;
 
 	/* Cached expiration times. */
 	sig->cputime_expires.prof_exp = cputime_zero;
diff --git a/kernel/itimer.c b/kernel/itimer.c
index ba66eac..4b2ba3b 100644
--- a/kernel/itimer.c
+++ b/kernel/itimer.c
@@ -44,12 +44,13 @@ static struct timeval itimer_get_remtime(struct hrtimer *timer)
 static void get_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 			   struct itimerval *value)
 {
-	cputime_t cval, cinterval;
+	cputime_t cval;
+	ktime_t kt_cinterval;
 
 	spin_lock_irq(&tsk->sighand->siglock);
 
 	cval = it->expires;
-	cinterval = it->incr;
+	kt_cinterval = it->incr;
 	if (!cputime_eq(cval, cputime_zero)) {
 		struct task_cputime cputime;
 		cputime_t utime;
@@ -66,7 +67,7 @@ static void get_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 	spin_unlock_irq(&tsk->sighand->siglock);
 
 	cputime_to_timeval(cval, &value->it_value);
-	cputime_to_timeval(cinterval, &value->it_interval);
+	value->it_interval = ktime_to_timeval(kt_cinterval);
 }
 
 int do_getitimer(int which, struct itimerval *value)
@@ -125,15 +126,16 @@ static void set_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 			   unsigned int clock_id, struct itimerval *value,
 			   struct itimerval *ovalue)
 {
-	cputime_t cval, cinterval, nval, ninterval;
+	cputime_t cval, nval;
+	ktime_t kt_cinterval, kt_ninterval;
 
 	nval = timeval_to_cputime(&value->it_value);
-	ninterval = timeval_to_cputime(&value->it_interval);
+	kt_ninterval = timeval_to_ktime(value->it_interval);
 
 	spin_lock_irq(&tsk->sighand->siglock);
 
 	cval = it->expires;
-	cinterval = it->incr;
+	kt_cinterval = it->incr;
 	if (!cputime_eq(cval, cputime_zero) ||
 	    !cputime_eq(nval, cputime_zero)) {
 		if (cputime_gt(nval, cputime_zero))
@@ -141,13 +143,13 @@ static void set_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 		set_process_cpu_timer(tsk, clock_id, &nval, &cval);
 	}
 	it->expires = nval;
-	it->incr = ninterval;
+	it->incr = kt_ninterval;
 
 	spin_unlock_irq(&tsk->sighand->siglock);
 
 	if (ovalue) {
 		cputime_to_timeval(cval, &ovalue->it_value);
-		cputime_to_timeval(cinterval, &ovalue->it_interval);
+		value->it_interval = ktime_to_timeval(kt_cinterval);
 	}
 }
 
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 654e655..9d6fdc9 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1077,9 +1077,27 @@ 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 (it->incr.tv64 != 0) {
+			ktime_t incr, real_incr, diff;
+			cputime_t cpu_incr;
+			struct timespec ts_incr, ts_real_incr;
+
+			incr = ktime_sub_ns(it->incr, it->err_ns);
+			if (unlikely(incr.tv64 <= 0))
+				incr = ktime_set(0, 1);
+
+			ts_incr = ktime_to_timespec(incr);
+			cpu_incr = timespec_to_cputime(&ts_incr);
+
+			cputime_to_timespec(cpu_incr, &ts_real_incr);
+			real_incr = timespec_to_ktime(ts_real_incr);
+
+			diff = ktime_sub(real_incr, incr);
+			it->err_ns = ktime_to_ns(diff);
+			it->expires = cputime_add(it->expires, cpu_incr);
+		} else {
+			it->expires = cputime_zero;
+			it->err_ns = 0;
 		}
 
 		__group_send_sig_info(signo, SEND_SIG_PRIV, tsk);
-- 
1.6.0.6