time/jiffies: Inline jiffies_to_msecs() and jiffies_to_usecs()

For common cases (HZ=100, 250 or 1000), these helpers are at most one
multiply, so there is no point calling a tiny function.

Keep them out of line for HZ=300 and others.

This saves cycles in TCP fast path, among other things.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/8 grow/shrink: 25/89 up/down: 530/-3474 (-2944)
...
nla_put_msecs                                193       -    -193
message_stats_print                         2131     920   -1211
Total: Before=25365208, After=25362264, chg -0.01%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260210170226.57209-1-edumazet@google.com

2 files changed, 45 insertions, 14 deletions

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index fdef2c155c27..d1c3d4941854 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -434,8 +434,44 @@ extern unsigned long preset_lpj;
 /*
  * Convert various time units to each other:
  */
-extern unsigned int jiffies_to_msecs(const unsigned long j);
-extern unsigned int jiffies_to_usecs(const unsigned long j);
+
+#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+/**
+ * jiffies_to_msecs - Convert jiffies to milliseconds
+ * @j: jiffies value
+ *
+ * This inline version takes care of HZ in {100,250,1000}.
+ *
+ * Return: milliseconds value
+ */
+static inline unsigned int jiffies_to_msecs(const unsigned long j)
+{
+	return (MSEC_PER_SEC / HZ) * j;
+}
+#else
+unsigned int jiffies_to_msecs(const unsigned long j);
+#endif
+
+#if !(USEC_PER_SEC % HZ)
+/**
+ * jiffies_to_usecs - Convert jiffies to microseconds
+ * @j: jiffies value
+ *
+ * Return: microseconds value
+ */
+static inline unsigned int jiffies_to_usecs(const unsigned long j)
+{
+	/*
+	 * Hz usually doesn't go much further MSEC_PER_SEC.
+	 * jiffies_to_usecs() and usecs_to_jiffies() depend on that.
+	 */
+	BUILD_BUG_ON(HZ > USEC_PER_SEC);
+
+	return (USEC_PER_SEC / HZ) * j;
+}
+#else
+unsigned int jiffies_to_usecs(const unsigned long j);
+#endif
 
 /**
  * jiffies_to_nsecs - Convert jiffies to nanoseconds
diff --git a/kernel/time/time.c b/kernel/time/time.c
index 0ba8e3c50d62..36fd2313ae7e 100644
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -365,20 +365,16 @@ SYSCALL_DEFINE1(adjtimex_time32, struct old_timex32 __user *, utp)
 }
 #endif
 
+#if HZ > MSEC_PER_SEC || (MSEC_PER_SEC % HZ)
 /**
  * jiffies_to_msecs - Convert jiffies to milliseconds
  * @j: jiffies value
  *
- * Avoid unnecessary multiplications/divisions in the
- * two most common HZ cases.
- *
  * Return: milliseconds value
  */
 unsigned int jiffies_to_msecs(const unsigned long j)
 {
-#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
-	return (MSEC_PER_SEC / HZ) * j;
-#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+#if HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
 	return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
 #else
 # if BITS_PER_LONG == 32
@@ -390,7 +386,9 @@ unsigned int jiffies_to_msecs(const unsigned long j)
 #endif
 }
 EXPORT_SYMBOL(jiffies_to_msecs);
+#endif
 
+#if (USEC_PER_SEC % HZ)
 /**
  * jiffies_to_usecs - Convert jiffies to microseconds
  * @j: jiffies value
@@ -405,17 +403,14 @@ unsigned int jiffies_to_usecs(const unsigned long j)
 	 */
 	BUILD_BUG_ON(HZ > USEC_PER_SEC);
 
-#if !(USEC_PER_SEC % HZ)
-	return (USEC_PER_SEC / HZ) * j;
-#else
-# if BITS_PER_LONG == 32
+#if BITS_PER_LONG == 32
 	return (HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32;
-# else
+#else
 	return (j * HZ_TO_USEC_NUM) / HZ_TO_USEC_DEN;
-# endif
 #endif
 }
 EXPORT_SYMBOL(jiffies_to_usecs);
+#endif
 
 /**
  * mktime64 - Converts date to seconds.