Re: [PATCH] Provide u64 version of jiffies_to_usecs() in kernel/tsacct.c

[Jonathan Lim]

It's possible that the values used in and returned from jiffies_to_usecs() are
incorrect because of truncation when variables of type u64 are involved.  So a
function specific to that type is used instead.

This version implements a correction to jiffies_64_to_usecs() based on feedback
from Randy Dunlap and Roman Zippel.

Diff'd against: linux/kernel/git/torvalds/linux-2.6.git

Signed-off-by: Jonathan Lim <jlim@xxxxxxx>

--- a/include/linux/jiffies.h	Thu Feb 14 18:04:14 PST 2008
+++ b/include/linux/jiffies.h	Thu Feb 14 18:07:17 PST 2008
@@ -42,7 +42,7 @@
 /* LATCH is used in the interval timer and ftape setup. */
 #define LATCH  ((CLOCK_TICK_RATE + HZ/2) / HZ)	/* For divider */
 
-/* Suppose we want to devide two numbers NOM and DEN: NOM/DEN, then we can
+/* Suppose we want to divide two numbers NOM and DEN: NOM/DEN, then we can
  * improve accuracy by shifting LSH bits, hence calculating:
  *     (NOM << LSH) / DEN
  * This however means trouble for large NOM, because (NOM << LSH) may no
@@ -204,7 +204,7 @@ extern unsigned long preset_lpj;
  * operator if the result is a long long AND at least one of the
  * operands is cast to long long (usually just prior to the "*" so as
  * not to confuse it into thinking it really has a 64-bit operand,
- * which, buy the way, it can do, but it takes more code and at least 2
+ * which, by the way, it can do, but it takes more code and at least 2
  * mpys).
 
  * We also need to be aware that one second in nanoseconds is only a
@@ -269,6 +269,7 @@ extern unsigned long preset_lpj;
  */
 extern unsigned int jiffies_to_msecs(const unsigned long j);
 extern unsigned int jiffies_to_usecs(const unsigned long j);
+extern u64 jiffies_64_to_usecs(const u64 j);
 extern unsigned long msecs_to_jiffies(const unsigned int m);
 extern unsigned long usecs_to_jiffies(const unsigned int u);
 extern unsigned long timespec_to_jiffies(const struct timespec *value);
--- a/kernel/time.c	Thu Feb 14 18:05:12 PST 2008
+++ b/kernel/time.c	Fri Apr 18 14:44:54 PDT 2008
@@ -38,6 +38,7 @@
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
+#include <asm/div64.h>
 
 #include "timeconst.h"
 
@@ -267,6 +268,16 @@ unsigned int inline jiffies_to_usecs(con
 #endif
 }
 EXPORT_SYMBOL(jiffies_to_usecs);
+
+u64 jiffies_64_to_usecs(const u64 j)
+{
+# if BITS_PER_LONG == 32
+	return ((u64)HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32;
+# else
+	return div64_64(j*HZ_TO_USEC_NUM + HZ_TO_USEC_DEN-1, HZ_TO_USEC_DEN);
+# endif
+}
+EXPORT_SYMBOL(jiffies_64_to_usecs);
 
 /**
  * timespec_trunc - Truncate timespec to a granularity
--- a/kernel/tsacct.c	Thu Feb 14 18:06:17 PST 2008
+++ b/kernel/tsacct.c	Thu Feb 14 18:08:47 PST 2008
@@ -85,8 +85,8 @@ void xacct_add_tsk(struct taskstats *sta
 	struct mm_struct *mm;
 
 	/* convert pages-jiffies to Mbyte-usec */
-	stats->coremem = jiffies_to_usecs(p->acct_rss_mem1) * PAGE_SIZE / MB;
-	stats->virtmem = jiffies_to_usecs(p->acct_vm_mem1) * PAGE_SIZE / MB;
+	stats->coremem = jiffies_64_to_usecs(p->acct_rss_mem1) * PAGE_SIZE / MB;
+	stats->virtmem = jiffies_64_to_usecs(p->acct_vm_mem1) * PAGE_SIZE / MB;
 	mm = get_task_mm(p);
 	if (mm) {
 		/* adjust to KB unit */
--
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