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Improve the inlining of jiffies_to_msecs() and jiffies_to_usecs(),

Browse source 
for the common HZ=100, 250 or 1000 cases, only inlining them
 for odd HZ values like HZ=300.
 
 The inlining overhead showed up in performance tests of the TCP code.
 
 (Marked as an RFC pull request, as it's not a regression.)
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'timers-urgent-2026-03-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer fix from Ingo Molnar:
 "Improve the inlining of jiffies_to_msecs() and jiffies_to_usecs(), for
  the common HZ=100, 250 or 1000 cases. Only use a function call for odd
  HZ values like HZ=300 that generate more code.

  The function call overhead showed up in performance tests of the TCP
  code"

* tag 'timers-urgent-2026-03-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  time/jiffies: Inline jiffies_to_msecs() and jiffies_to_usecs()
This commit is contained in:
Linus Torvalds 2026-03-01 12:15:58 -08:00
commit f6542af922
2 changed files with 45 additions and 14 deletions
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40
include/linux/jiffies.h
View file

@ -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

19
kernel/time/time.c
View file

@ -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.