[PATCH v2 0/2] x86/topology: add bitmap_weight_from() and use it in topo_unit_count()

[PATCH v2 0/2] x86/topology: add bitmap_weight_from() and use it in topo_unit_count()

[Yury Norov (NVIDIA)]

Currently the function open-codes the missing bitmap_weight_from() with
a for-loop, which is known to be less effective. Add the helper and
switch topo_unit_count() to using it.

v1: https://lore.kernel.org/all/20251214235437.244125-1-yury.norov@gmail.com/
v2:
 - rename 'nbits' to 'end' (Ingo);
 - leverage bitmap_weight() in inline implementation;
 - drop outline implementation;
 - don't handle 'start >= end' case explititly, leave it to user.

Yury Norov (NVIDIA) (2):
  bitmap: add bitmap_weight_from()
  x86/topology: use bitmap_weight_from()

 arch/x86/kernel/cpu/topology.c |  8 ++------
 include/linux/bitmap.h         | 30 ++++++++++++++++++++++++++++++
 lib/test_bitmap.c              | 26 ++++++++++++++++++++++++++
 3 files changed, 58 insertions(+), 6 deletions(-)

-- 
2.43.0

[PATCH v2 1/2] bitmap: add bitmap_weight_from()

[Yury Norov (NVIDIA)]

The function calculates a Hamming weight of a bitmap starting from an
arbitrary bit.

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
 include/linux/bitmap.h | 30 ++++++++++++++++++++++++++++++
 lib/test_bitmap.c      | 26 ++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index b0395e4ccf90..fdff6e87512b 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -57,6 +57,7 @@ struct device;
  *  bitmap_weight(src, nbits)                   Hamming Weight: number set bits
  *  bitmap_weight_and(src1, src2, nbits)        Hamming Weight of and'ed bitmap
  *  bitmap_weight_andnot(src1, src2, nbits)     Hamming Weight of andnot'ed bitmap
+ *  bitmap_weight_from(src, start, end)         Hamming Weight starting from @start
  *  bitmap_set(dst, pos, nbits)                 Set specified bit area
  *  bitmap_clear(dst, pos, nbits)               Clear specified bit area
  *  bitmap_find_next_zero_area(buf, len, pos, n, mask)  Find bit free area
@@ -479,6 +480,35 @@ unsigned long bitmap_weight_andnot(const unsigned long *src1,
 	return __bitmap_weight_andnot(src1, src2, nbits);
 }
 
+/**
+ * bitmap_weight_from - Hamming weight for a memory region
+ * @bitmap: The base address
+ * @start: The bitnumber to starts weighting
+ * @end: the bitmap size in bits
+ *
+ * Returns the number of set bits in the region. If @start >= @end,
+ * the result is undefined.
+ */
+static __always_inline
+unsigned long bitmap_weight_from(const unsigned long *bitmap,
+				   unsigned int start, unsigned int end)
+{
+	unsigned long w;
+
+	if (small_const_nbits(end))
+		return hweight_long(*bitmap & GENMASK(end - 1, start));
+
+	bitmap += start / BITS_PER_LONG;
+	/* Opencode round_down() to not include math.h */
+	end -= start & ~(BITS_PER_LONG - 1);
+	start %= BITS_PER_LONG;
+	w = bitmap_weight(bitmap, end);
+	if (start)
+		w -= hweight_long(*bitmap & BITMAP_LAST_WORD_MASK(start));
+
+	return w;
+}
+
 static __always_inline
 void bitmap_set(unsigned long *map, unsigned int start, unsigned int nbits)
 {
diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
index c83829ef557f..eff0b1eba8e4 100644
--- a/lib/test_bitmap.c
+++ b/lib/test_bitmap.c
@@ -851,6 +851,31 @@ static void __init test_for_each_set_bit_from(void)
 	}
 }
 
+static void __init test_bitmap_weight(void)
+{
+	unsigned int bit, w1, w2, w;
+	DECLARE_BITMAP(b, 30);
+
+	bitmap_parselist("all:1/2", b, 30);
+
+	/* Test inline implementation */
+	w = bitmap_weight(b, 30);
+	w1 = bitmap_weight(b, 15);
+	w2 = bitmap_weight_from(b, 15, 30);
+
+	expect_eq_uint(15, w);
+	expect_eq_uint(8, w1);
+	expect_eq_uint(7, w2);
+
+	/* Test outline implementation */
+	w = bitmap_weight(exp1, EXP1_IN_BITS);
+	for (bit = 0; bit < EXP1_IN_BITS; bit++) {
+		w1 = bitmap_weight(exp1, bit);
+		w2 = bitmap_weight_from(exp1, bit, EXP1_IN_BITS);
+		expect_eq_uint(w1 + w2, w);
+	}
+}
+
 static void __init test_for_each_clear_bit(void)
 {
 	DECLARE_BITMAP(orig, 500);
@@ -1441,6 +1466,7 @@ static void __init selftest(void)
 	test_bitmap_const_eval();
 	test_bitmap_read_write();
 	test_bitmap_read_perf();
+	test_bitmap_weight();
 	test_bitmap_write_perf();
 
 	test_find_nth_bit();
-- 
2.43.0

[PATCH v2 2/2] x86/topology: use bitmap_weight_from()

[Yury Norov (NVIDIA)]

Switch topo_unit_count() to use bitmap_weight_from().

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
 arch/x86/kernel/cpu/topology.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index f55ea3cdbf88..ba7142f417ad 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -204,15 +204,11 @@ static __init bool check_for_real_bsp(u32 apic_id)
 static unsigned int topo_unit_count(u32 lvlid, enum x86_topology_domains at_level,
 				    unsigned long *map)
 {
-	unsigned int id, end, cnt = 0;
+	unsigned int end;
 
 	/* Calculate the exclusive end */
 	end = lvlid + (1U << x86_topo_system.dom_shifts[at_level]);
-
-	/* Unfortunately there is no bitmap_weight_range() */
-	for (id = find_next_bit(map, end, lvlid); id < end; id = find_next_bit(map, end, ++id))
-		cnt++;
-	return cnt;
+	return bitmap_weight_from(map, lvlid, end);
 }
 
 static __init void topo_register_apic(u32 apic_id, u32 acpi_id, bool present)
-- 
2.43.0

Re: [PATCH v2 0/2] x86/topology: add bitmap_weight_from() and use it in topo_unit_count()

[Yury Norov]

Ping?

On Mon, Dec 22, 2025 at 02:11:36PM -0500, Yury Norov (NVIDIA) wrote:
> Currently the function open-codes the missing bitmap_weight_from() with
> a for-loop, which is known to be less effective. Add the helper and
> switch topo_unit_count() to using it.
> 
> v1: https://lore.kernel.org/all/20251214235437.244125-1-yury.norov@gmail.com/
> v2:
>  - rename 'nbits' to 'end' (Ingo);
>  - leverage bitmap_weight() in inline implementation;
>  - drop outline implementation;
>  - don't handle 'start >= end' case explititly, leave it to user.
> 
> Yury Norov (NVIDIA) (2):
>   bitmap: add bitmap_weight_from()
>   x86/topology: use bitmap_weight_from()
> 
>  arch/x86/kernel/cpu/topology.c |  8 ++------
>  include/linux/bitmap.h         | 30 ++++++++++++++++++++++++++++++
>  lib/test_bitmap.c              | 26 ++++++++++++++++++++++++++
>  3 files changed, 58 insertions(+), 6 deletions(-)
> 
> -- 
> 2.43.0

Re: [PATCH v2 0/2] x86/topology: add bitmap_weight_from() and use it in topo_unit_count()

[Yury Norov]

On Sun, Feb 22, 2026 at 12:49:51PM -0500, Yury Norov wrote:
> Ping?

OK, if no objections, I'm taking it in bitmap-for-next.

Thanks,
Yury
 
> On Mon, Dec 22, 2025 at 02:11:36PM -0500, Yury Norov (NVIDIA) wrote:
> > Currently the function open-codes the missing bitmap_weight_from() with
> > a for-loop, which is known to be less effective. Add the helper and
> > switch topo_unit_count() to using it.
> > 
> > v1: https://lore.kernel.org/all/20251214235437.244125-1-yury.norov@gmail.com/
> > v2:
> >  - rename 'nbits' to 'end' (Ingo);
> >  - leverage bitmap_weight() in inline implementation;
> >  - drop outline implementation;
> >  - don't handle 'start >= end' case explititly, leave it to user.
> > 
> > Yury Norov (NVIDIA) (2):
> >   bitmap: add bitmap_weight_from()
> >   x86/topology: use bitmap_weight_from()
> > 
> >  arch/x86/kernel/cpu/topology.c |  8 ++------
> >  include/linux/bitmap.h         | 30 ++++++++++++++++++++++++++++++
> >  lib/test_bitmap.c              | 26 ++++++++++++++++++++++++++
> >  3 files changed, 58 insertions(+), 6 deletions(-)
> > 
> > -- 
> > 2.43.0