Re: [PATCH] time/jiffies: inline jiffies_to_msecs() and jiffies_to_usecs()

[Thomas Huth]

On 10/02/2026 18.02, Eric Dumazet wrote:
> For common cases (HZ=100, 250 or 1000), these helpers are
> at most one multiply, 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@xxxxxxxxxx>
> ---
>   include/linux/jiffies.h | 40 ++++++++++++++++++++++++++++++++++++++--
>   kernel/time/time.c      | 19 +++++++------------
>   2 files changed, 45 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
> index fdef2c155c27202ae8ba910f170a62521294dc36..d1c3d4941854a82c4571032adb41ca57d5faa8bd 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
> + */
 Hi Eric!

This now causes warnings when doing "make htmldocs":

.../Documentation/driver-api/basics:42: .../kernel/time/time.c:387: WARNING: 
Duplicate C declaration, also defined at driver-api/basics:436.
Declaration is '.. c:function:: unsigned int jiffies_to_msecs (const 
unsigned long j)'. [duplicate_declaration.c]
.../Documentation/driver-api/basics:42: .../kernel/time/time.c:410: WARNING: 
Duplicate C declaration, also defined at driver-api/basics:453.
Declaration is '.. c:function:: unsigned int jiffies_to_usecs (const 
unsigned long j)'. [duplicate_declaration.c]

Any ideas how to fix that best? Maybe the comments in the header should be 
just normal comments, and the kerneldoc should just reside in time.c ?

 Thanks,
  Thomas