Utilize "__fls()" in replacement of while loop counting
for the decremenet of "sftacc". They're equivalent in computation result
but the former is more effective.

"__fls()" will return the bit number of the last set bit of
"tmp", which is 0-based index. Plus 1 to convert it into bit width as
desired.

Note that only the lowest 32 bits of "tmp" is taken into consideration
of the operation, since it was already shifted right by 32 bits, the
topmost 32 bits should remain 0, only the lowest 32 bits are possible to
be non-zero.

This change is tested against a test script [1].
Run the test 10 times for each version of implementation and take the
average. The result shown that with this change, the operation overhead
of "clocks_calc_mult_shift()" can be reduced around 99.7% .

-----------------------------
| old version | new version |
-----------------------------
|  11500.6 ns |       44 ns |
-----------------------------

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
---
Changelog:

v1 -> v2:
	- Refine commit message to explain more about "why"
	- Check the frequency of "clocks_calc_mult_shift()" get called,
	  it's not in hotpath on my machine, refine the commit message
to avoid overselling it
	- Add comments for the code to explain the implementation in
	  more detail
	- Handle case for "tmp == 0" to avoid undefined behavior

v2 -> v3:
	- Use "find_last_bit()" instead of "__builtin_clz()"
	- Convert the type of "tmp" to "const unsigned long *" when
	  sending into the function
	- Highlight in the comment that only the lowest 32 bits part
	  of "tmp" is taken into consideration

v3 -> v4:
	- Use "__fls()" since "tmp" is of type u64, not cpumask
	- Refine commit messages to match the current implementation

v4 -> v5:
	- Update commit header to mention the use of __fls()

[1]:
static int __init test_init(void)
{
    u32 mult, shift;
    u32 from, to, maxsec;
    ktime_t start_time, end_time, total_time;
    pr_info("Starting clocks_calc_mult_shift simple test\n");

    start_time = ktime_get();
    // Test with parameters from 1 to 1000
    for (from = 1; from <= 1000; from += 100) {
        for (to = 1; to <= 1000; to += 100) {
            for (maxsec = 1; maxsec <= 10; maxsec++) {

                clocks_calc_mult_shift(&mult, &shift, from, to, maxsec);
            }
        }
    }

    end_time = ktime_get();
    total_time = ktime_to_ns(ktime_sub(end_time, start_time));

    pr_info("Test completed\n");
    pr_info("Total execution time: %lld ns \n", total_time);
    return 0;
}

The test is running in the form of kernel module.
The test machine is running ubuntu 24.04 on x86_64 machine with kernel
version of v6.14.0, CPU type is AMD Ryzen 7 5700X3D 8-Core Processor.

Best regards,
I Hsin Cheng.
---
 kernel/time/clocksource.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 2a7802ec480c..1e3dc68c696d 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -66,10 +66,19 @@ clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 maxsec)
 	 * range:
 	 */
 	tmp = ((u64)maxsec * from) >> 32;
-	while (tmp) {
-		tmp >>=1;
-		sftacc--;
-	}
+
+	/*
+	 * Decrement "sftacc" by the number of bits needed to represent "tmp".
+	 * Using "__fls(tmp) + 1" to get the bit width:
+	 * - __fls(tmp) returns the bit number of the last set bit
+	 * - Plus 1 to convert 0-based index into bit width as desired
+	 *
+	 * Note: Only the lowest 32 bits of "tmp" is taken into consideration,
+	 *		 since it was already shifted right by 32 bits, the topmost 32
+	 *		 bits are guaranteed to be 0.
+	 */
+	if (tmp)
+		sftacc -= __fls(tmp) + 1;
 
 	/*
 	 * Find the conversion shift/mult pair which has the best
-- 
2.43.0

On Fri, Jun 13, 2025 at 11:52:39AM +0800, I Hsin Cheng wrote:
> Utilize "__fls()" in replacement of while loop counting
> for the decremenet of "sftacc". They're equivalent in computation result
> but the former is more effective.
> 
> "__fls()" will return the bit number of the last set bit of
> "tmp", which is 0-based index. Plus 1 to convert it into bit width as
> desired.
> 
> Note that only the lowest 32 bits of "tmp" is taken into consideration
> of the operation, since it was already shifted right by 32 bits, the
> topmost 32 bits should remain 0, only the lowest 32 bits are possible to
> be non-zero.
> 
> This change is tested against a test script [1].

And because [1] is in temporary section, git readers will have no chance
to follow it.

> Run the test 10 times for each version of implementation and take the
> average. The result shown that with this change, the operation overhead
> of "clocks_calc_mult_shift()" can be reduced around 99.7% .
> 
> -----------------------------
> | old version | new version |
> -----------------------------
> |  11500.6 ns |       44 ns |
> -----------------------------

44 ns is more or less minimal time delay that a typical x86_64 machine
is able to measure. What you have measured on 'new' side is pretty
likely a single timer tick, maybe two. The reliable time intervals for
performance measurements are within few milliseconds.

> Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
> ---
> Changelog:
> 
> v1 -> v2:
> 	- Refine commit message to explain more about "why"
> 	- Check the frequency of "clocks_calc_mult_shift()" get called,
> 	  it's not in hotpath on my machine, refine the commit message
> to avoid overselling it
> 	- Add comments for the code to explain the implementation in
> 	  more detail
> 	- Handle case for "tmp == 0" to avoid undefined behavior
> 
> v2 -> v3:
> 	- Use "find_last_bit()" instead of "__builtin_clz()"
> 	- Convert the type of "tmp" to "const unsigned long *" when
> 	  sending into the function
> 	- Highlight in the comment that only the lowest 32 bits part
> 	  of "tmp" is taken into consideration
> 
> v3 -> v4:
> 	- Use "__fls()" since "tmp" is of type u64, not cpumask
> 	- Refine commit messages to match the current implementation
> 
> v4 -> v5:
> 	- Update commit header to mention the use of __fls()
> 
> [1]:
> static int __init test_init(void)
> {
>     u32 mult, shift;
>     u32 from, to, maxsec;
>     ktime_t start_time, end_time, total_time;
>     pr_info("Starting clocks_calc_mult_shift simple test\n");
> 
>     start_time = ktime_get();
>     // Test with parameters from 1 to 1000
>     for (from = 1; from <= 1000; from += 100) {
>         for (to = 1; to <= 1000; to += 100) {
>             for (maxsec = 1; maxsec <= 10; maxsec++) {
> 
>                 clocks_calc_mult_shift(&mult, &shift, from, to, maxsec);
>             }
>         }
>     }
> 
>     end_time = ktime_get();
>     total_time = ktime_to_ns(ktime_sub(end_time, start_time));
> 
>     pr_info("Test completed\n");
>     pr_info("Total execution time: %lld ns \n", total_time);
>     return 0;
> }
> 
> The test is running in the form of kernel module.
> The test machine is running ubuntu 24.04 on x86_64 machine with kernel
> version of v6.14.0, CPU type is AMD Ryzen 7 5700X3D 8-Core Processor.

x86 has the accelerated __fls(), so employing it broadly is a
non-questionable improvement. I don't see any benefit in posting the
snippets of that sort. Does somebody doubt that one wins over another?

The problem of clocks_calc_mult_shift() is that it opencodes the
existing helper, not that it does that by using a naive shift approach.
It may not be a performance problem if it's not a hot path, but it's
for sure a sort of coding culture problem.

If you want to measure accelerated __fls() vs generic___fls() vs this
naive __fls() performance, it may be an interesting exercise, but
definitely out of the scope of clocksource improvement.

If you want to do that, I'd suggest to extend the find_bit_benchmark
test.

> Best regards,
> I Hsin Cheng.
> ---
>  kernel/time/clocksource.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 2a7802ec480c..1e3dc68c696d 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -66,10 +66,19 @@ clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 maxsec)
>  	 * range:
>  	 */
>  	tmp = ((u64)maxsec * from) >> 32;
> -	while (tmp) {
> -		tmp >>=1;
> -		sftacc--;
> -	}
> +
> +	/*
> +	 * Decrement "sftacc" by the number of bits needed to represent "tmp".
> +	 * Using "__fls(tmp) + 1" to get the bit width:
> +	 * - __fls(tmp) returns the bit number of the last set bit
> +	 * - Plus 1 to convert 0-based index into bit width as desired

Please don't explain how, just explain why.

> +	 *
> +	 * Note: Only the lowest 32 bits of "tmp" is taken into consideration,
> +	 *		 since it was already shifted right by 32 bits, the topmost 32
> +	 *		 bits are guaranteed to be 0.

Then tmp should be u32, right?

I think compiler would warn about implicit 64-to-32 typecast for
32-bit targets, which would be a false positive in this case.

> +	 */
> +	if (tmp)
> +		sftacc -= __fls(tmp) + 1;

Suggested-by: Yury Norov [NVIDIA] <yury.norov@nvidia.com> # for __fls()

>  
>  	/*
>  	 * Find the conversion shift/mult pair which has the best
> -- 
> 2.43.0