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


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