[PATCH resend3 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>
---
Compared to previous patch bug (using value->it_interval instead of
ovalue->it_interavl) is fixed.

 include/linux/sched.h     |    3 ++-
 kernel/fork.c             |    6 ++++--
 kernel/itimer.c           |   23 +++++++++++++----------
 kernel/posix-cpu-timers.c |   24 +++++++++++++++++++++---
 4 files changed, 40 insertions(+), 16 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0d8367b..f01ae49 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;
 };
 
 /**
diff --git a/kernel/fork.c b/kernel/fork.c
index fbde8e0..325ebd9 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[0].expires = cputime_zero;
-	sig->it[0].incr = cputime_zero;
+	sig->it[0].incr = ktime_set(0, 0);
+	sig->it[0].err_ns = 0;
 	sig->it[1].expires = cputime_zero;
-	sig->it[1].incr = cputime_zero;
+	sig->it[1].incr = ktime_set(0, 0);
+	sig->it[1].err_ns = 0;
 
 	/* Cached expiration times. */
 	sig->cputime_expires.prof_exp = cputime_zero;
diff --git a/kernel/itimer.c b/kernel/itimer.c
index bb8bde6..5930d94 100644
--- a/kernel/itimer.c
+++ b/kernel/itimer.c
@@ -42,15 +42,16 @@ 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;
+	cputime_t cval;
+	ktime_t kt_cinterval;
 	struct cpu_itimer *it = &tsk->signal->it[clock_id];
 
 	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 t;
@@ -77,7 +78,7 @@ static void get_cpu_itimer(struct task_struct *tsk, unsigned int clock_id,
 	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)
@@ -133,18 +134,20 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
 }
 
 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;
+	ktime_t kt_cinterval, kt_ninterval;
 	struct cpu_itimer *it = &tsk->signal->it[clock_id];
 
 	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))
@@ -152,13 +155,13 @@ static void set_cpu_itimer(struct task_struct *tsk, unsigned int clock_id,
 		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);
+		ovalue->it_interval = ktime_to_timeval(kt_cinterval);
 	}
 }
 
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index b6accca..905734b 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1080,9 +1080,27 @@ 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