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>

1 files changed, 21 insertions, 3 deletions

diff --git a/kernel/itimer.c b/kernel/itimer.c
index 852c88ddd1f0..21adff7b2a17 100644
--- 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);