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

[Stanislaw Gruszka]

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@xxxxxxxxxx>
---
 include/linux/sched.h     |    3 ++-
 kernel/fork.c             |    6 ++++--
 kernel/itimer.c           |   21 +++++++++++++--------
 kernel/posix-cpu-timers.c |   24 +++++++++++++++++++++---
 4 files changed, 40 insertions(+), 14 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b4c19c2..c6e1946 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -529,7 +529,8 @@ struct signal_struct {
 
 	/* ITIMER_PROF and ITIMER_VIRTUAL timers for the process */
 	cputime_t it_expires[2];
-	cputime_t it_incr[2];
+	ktime_t it_incr[2];
+	u32 it_err_ns[2];
 
 	/*
 	 * Thread group totals for process CPU timers.
diff --git a/kernel/fork.c b/kernel/fork.c
index 6c7044a..b1bb3e4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -805,8 +805,10 @@ static void posix_cpu_timers_init_group(struct signal_struct *sig)
 	/* Expiration times and increments. */
 	sig->it_expires[IT_VIRT] = cputime_zero;
 	sig->it_expires[IT_PROF] = cputime_zero;
-	sig->it_incr[IT_VIRT] = cputime_zero;
-	sig->it_incr[IT_PROF] = cputime_zero;
+	sig->it_incr[IT_VIRT] = ktime_set(0, 0);
+	sig->it_incr[IT_PROF] = ktime_set(0, 0);
+	sig->it_err_ns[IT_VIRT] = 0;
+	sig->it_err_ns[IT_PROF] = 0;
 
 	/* Cached expiration times. */
 	sig->cputime_expires.prof_exp = cputime_zero;
diff --git a/kernel/itimer.c b/kernel/itimer.c
index ec64fea..ad06bd5 100644
--- a/kernel/itimer.c
+++ b/kernel/itimer.c
@@ -44,11 +44,13 @@ static struct timeval itimer_get_remtime(struct hrtimer *timer)
 static void get_cpu_itimer(struct task_struct *tsk, enum itimer_number i,
 			   struct itimerval *value)
 {
-	cputime_t cval, cinterval;
+	cputime_t cval;
+	ktime_t kt_cinterval;
 
 	spin_lock_irq(&tsk->sighand->siglock);
+
 	cval = tsk->signal->it_expires[i];
-	cinterval = tsk->signal->it_incr[i];
+	kt_cinterval = tsk->signal->it_incr[i];
 	if (!cputime_eq(cval, cputime_zero)) {
 		struct task_cputime cputime;
 		cputime_t utime;
@@ -61,9 +63,11 @@ static void get_cpu_itimer(struct task_struct *tsk, enum itimer_number i,
 			cval = cputime_sub(cval, utime);
 		}
 	}
+
 	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)
@@ -121,15 +125,16 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
 static void set_cpu_itimer(struct task_struct *tsk, enum itimer_number i,
 			   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 = tsk->signal->it_expires[i];
-	cinterval = tsk->signal->it_incr[i];
+	kt_cinterval = tsk->signal->it_incr[i];
 	if (!cputime_eq(cval, cputime_zero) ||
 	    !cputime_eq(nval, cputime_zero)) {
 		if (cputime_gt(nval, cputime_zero))
@@ -137,13 +142,13 @@ static void set_cpu_itimer(struct task_struct *tsk, enum itimer_number i,
 		set_process_cpu_timer(tsk, i, &nval, &cval);
 	}
 	tsk->signal->it_expires[i] = nval;
-	tsk->signal->it_incr[i] = ninterval;
+	tsk->signal->it_incr[i] = 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 76e125c..9c3ea39 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1080,10 +1080,28 @@ static cputime_t check_cpu_itimer(struct task_struct *tsk, enum itimer_number i,
 	if (cputime_ge(cur_time, sig->it_expires[i])) {
 		int signo;
 
-		sig->it_expires[i] = sig->it_incr[i];
-		if (!cputime_eq(sig->it_expires[i], cputime_zero)) {
+		if (sig->it_incr[i].tv64 != 0) {
+			ktime_t incr, real_incr, diff;
+			cputime_t cpu_incr;
+			struct timespec ts_incr, ts_real_incr;
+
+			incr = ktime_sub_ns(sig->it_incr[i], sig->it_err_ns[i]);
+			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);
+			sig->it_err_ns[i] = ktime_to_ns(diff);
 			sig->it_expires[i] = cputime_add(sig->it_expires[i],
-							 cur_time);
+							 cpu_incr);
+		} else {
+			sig->it_expires[i] = cputime_zero;
+			sig->it_err_ns[i] = 0;
 		}
 
 		signo = (i == IT_VIRT) ? SIGVTALRM : SIGPROF;
-- 
1.6.0.6

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