[PATCH -tip 3/3] define nsecs_to_jiffies()

[Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>]

Use of msecs_to_jiffies() for nsecs_to_cputime() have some problems:

 - The type of msecs_to_jiffies()'s argument is unsigned int, so
   it cannot convert msecs greater than UINT_MAX = about 49.7 days.
 - msecs_to_jiffies() returns MAX_JIFFY_OFFSET if MSB of argument
   is set, assuming that input was negative value.  So it cannot
   convert msecs greater than INT_MAX = about 24.8 days too.

This patch defines a new function nsecs_to_jiffies() that can deal
greater values, and that can deal all incoming values as unsigned.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
---
 include/linux/jiffies.h |    1 +
 kernel/sched.c          |    3 +--
 kernel/time.c           |   30 ++++++++++++++++++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 1a9cf78..6811f4b 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -307,6 +307,7 @@ extern clock_t jiffies_to_clock_t(long x);
 extern unsigned long clock_t_to_jiffies(unsigned long x);
 extern u64 jiffies_64_to_clock_t(u64 x);
 extern u64 nsec_to_clock_t(u64 x);
+extern unsigned long nsecs_to_jiffies(u64 n);
 
 #define TIMESTAMP_SIZE	30
 
diff --git a/kernel/sched.c b/kernel/sched.c
index a7093c1..792b25e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5156,8 +5156,7 @@ void task_times(struct task_struct *p, cputime_t *ut, cputime_t *st)
 #else
 
 #ifndef nsecs_to_cputime
-# define nsecs_to_cputime(__nsecs) \
-	msecs_to_cputime(div_u64((__nsecs), NSEC_PER_MSEC))
+# define nsecs_to_cputime(__nsecs)	nsecs_to_jiffies(__nsecs)
 #endif
 
 void task_times(struct task_struct *p, cputime_t *ut, cputime_t *st)
diff --git a/kernel/time.c b/kernel/time.c
index 2e2e469..b30016c 100644
--- a/kernel/time.c
+++ b/kernel/time.c
@@ -662,6 +662,36 @@ u64 nsec_to_clock_t(u64 x)
 #endif
 }
 
+/**
+ * nsecs_to_jiffies - Convert nsecs in u64 to jiffies
+ *
+ * @n:	nsecs in u64
+ *
+ * Unlike {m,u}secs_to_jiffies, type of input is not unsigned int but u64.
+ * And this doesn't return MAX_JIFFY_OFFSET since this function is designed
+ * for scheduler, not for use in device drivers to calculate timeout value.
+ *
+ * note:
+ *   NSEC_PER_SEC = 10^9 = (5^9 * 2^9) = (1953125 * 512)
+ *   ULLONG_MAX ns = 18446744073.709551615 secs = about 584 years
+ */
+unsigned long nsecs_to_jiffies(u64 n)
+{
+#if (NSEC_PER_SEC % HZ) == 0
+	/* Common case, HZ = 100, 128, 200, 250, 256, 500, 512, 1000 etc. */
+	return div_u64(n, NSEC_PER_SEC / HZ);
+#elif (HZ % 512) == 0
+	/* overflow after 292 years if HZ = 1024 */
+	return div_u64(n * HZ / 512, NSEC_PER_SEC / 512);
+#else
+	/*
+	 * Generic case - optimized for cases where HZ is a multiple of 3.
+	 * overflow after 64.99 years, exact for HZ = 60, 72, 90, 120 etc.
+	 */
+	return div_u64(n * 9, (9ull * NSEC_PER_SEC + HZ / 2) / HZ);
+#endif
+}
+
 #if (BITS_PER_LONG < 64)
 u64 get_jiffies_64(void)
 {
-- 
1.6.5.3