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 <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 17 insertions, 3 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9b2d5e4dc8c4..b60d644ea4b3 100644
--- 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);