[PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Kuan-Wei Chiu]

This patch series introduces equality-aware variants of the min heap
API that use a top-down heapify strategy to improve performance when
many elements are equal under the comparison function. It also updates
the documentation accordingly and modifies bcache to use the new APIs
to fix a performance regression caused by the switch to the generic min
heap library.

In particular, invalidate_buckets_lru() in bcache suffered from
increased comparison overhead due to the bottom-up strategy introduced
in commit 866898efbb25 ("bcache: remove heap-related macros and switch
to generic min_heap"). The regression is addressed by switching to the
equality-aware variants and using the inline versions to avoid function
call overhead in this hot path.

Cc: stable@vger.kernel.org
---

To avoid duplicated effort and expedite resolution, Robert kindly
agreed that I should submit my already-completed series instead. Many
thanks to him for his cooperation and support.

Kuan-Wei Chiu (8):
  lib min_heap: Add equal-elements-aware sift_down variant
  lib min_heap: Add typedef for sift_down function pointer
  lib min_heap: add eqaware variant of min_heapify_all()
  lib min_heap: add eqaware variant of min_heap_pop()
  lib min_heap: add eqaware variant of min_heap_pop_push()
  lib min_heap: add eqaware variant of min_heap_del()
  Documentation/core-api: min_heap: Document _eqaware variants of
    min-heap APIs
  bcache: Fix the tail IO latency regression by using equality-aware min
    heap API

 Documentation/core-api/min_heap.rst |  20 +++++
 drivers/md/bcache/alloc.c           |  15 ++--
 include/linux/min_heap.h            | 131 +++++++++++++++++++++++-----
 lib/min_heap.c                      |  23 +++--
 4 files changed, 154 insertions(+), 35 deletions(-)

-- 
2.34.1

[PATCH 1/8] lib min_heap: Add equal-elements-aware sift_down variant

[Kuan-Wei Chiu]

The existing min_heap_sift_down() uses the bottom-up heapify variant,
which reduces the number of comparisons from ~2 * log2(n) to
~1 * log2(n) when all elements are distinct. However, in workloads
where the heap contains many equal elements, this bottom-up variant
can degenerate and perform up to 2 * log2(n) comparisons, while the
traditional top-down variant needs only O(1) comparisons in such cases.

To address this, introduce min_heap_sift_down_eqaware(), a top-down
heapify variant optimized for scenarios with many equal elements. This
variant avoids unnecessary comparisons and swaps when elements are
already equal or in the correct position.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/min_heap.h | 51 ++++++++++++++++++++++++++++++++++++++++
 lib/min_heap.c           |  7 ++++++
 2 files changed, 58 insertions(+)

diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
index 79ddc0adbf2b..b0d603fe5379 100644
--- a/include/linux/min_heap.h
+++ b/include/linux/min_heap.h
@@ -292,6 +292,52 @@ void __min_heap_sift_down_inline(min_heap_char *heap, size_t pos, size_t elem_si
 	__min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos,	\
 				    __minheap_obj_size(_heap), _func, _args)
 
+/*
+ * Sift the element at pos down the heap.
+ *
+ * Variants of heap functions using an equal-elements-aware sift_down.
+ * These may perform better when the heap contains many equal elements.
+ */
+static __always_inline
+void __min_heap_sift_down_eqaware_inline(min_heap_char * heap, size_t pos, size_t elem_size,
+					 const struct min_heap_callbacks *func, void *args)
+{
+	void *data = heap->data;
+	void (*swp)(void *lhs, void *rhs, void *args) = func->swp;
+	/* pre-scale counters for performance */
+	size_t a = pos * elem_size;
+	size_t b, c, smallest;
+	size_t n = heap->nr * elem_size;
+
+	if (!swp)
+		swp = select_swap_func(data, elem_size);
+
+	for (;;) {
+		b = 2 * a + elem_size;
+		c = b + elem_size;
+		smallest = a;
+
+		if (b >= n)
+			break;
+
+		if (func->less(data + b, data + smallest, args))
+			smallest = b;
+
+		if (c < n && func->less(data + c, data + smallest, args))
+			smallest = c;
+
+		if (smallest == a)
+			break;
+
+		do_swap(data + a, data + smallest, elem_size, swp, args);
+		a = smallest;
+	}
+}
+
+#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)	\
+	__min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos,	\
+				    __minheap_obj_size(_heap), _func, _args)
+
 /* Sift up ith element from the heap, O(log2(nr)). */
 static __always_inline
 void __min_heap_sift_up_inline(min_heap_char *heap, size_t elem_size, size_t idx,
@@ -433,6 +479,8 @@ void *__min_heap_peek(struct min_heap_char *heap);
 bool __min_heap_full(min_heap_char *heap);
 void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
 			  const struct min_heap_callbacks *func, void *args);
+void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
+				  const struct min_heap_callbacks *func, void *args);
 void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
 			const struct min_heap_callbacks *func, void *args);
 void __min_heapify_all(min_heap_char *heap, size_t elem_size,
@@ -455,6 +503,9 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
 #define min_heap_sift_down(_heap, _pos, _func, _args)	\
 	__min_heap_sift_down(container_of(&(_heap)->nr, min_heap_char, nr), _pos,	\
 			     __minheap_obj_size(_heap), _func, _args)
+#define min_heap_sift_down_eqaware(_heap, _pos, _func, _args)	\
+	__min_heap_sift_down_eqaware(container_of(&(_heap)->nr, min_heap_char, nr), _pos,	\
+			     __minheap_obj_size(_heap), _func, _args)
 #define min_heap_sift_up(_heap, _idx, _func, _args)	\
 	__min_heap_sift_up(container_of(&(_heap)->nr, min_heap_char, nr),	\
 			   __minheap_obj_size(_heap), _idx, _func, _args)
diff --git a/lib/min_heap.c b/lib/min_heap.c
index 96f01a4c5fb6..2225f40d0d7a 100644
--- a/lib/min_heap.c
+++ b/lib/min_heap.c
@@ -27,6 +27,13 @@ void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
 }
 EXPORT_SYMBOL(__min_heap_sift_down);
 
+void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
+				  const struct min_heap_callbacks *func, void *args)
+{
+	__min_heap_sift_down_eqaware_inline(heap, pos, elem_size, func, args);
+}
+EXPORT_SYMBOL(__min_heap_sift_down_eqaware);
+
 void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
 			const struct min_heap_callbacks *func, void *args)
 {
-- 
2.34.1

Re: [PATCH 1/8] lib min_heap: Add equal-elements-aware sift_down variant

[Robert Pang]

Hi Kuan-Wei

Thanks for this patch series to address the bcache latency regression.
I tested it but results show regression still remains. Upon review of
the patch changes, I notice that the min_heap_sift_down_eqaware_inline
#define macro in this patch may have been mapped incorrectly:

+#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)   \
+       __min_heap_sift_down_inline(container_of(&(_heap)->nr,
min_heap_char, nr), _pos,        \
+                                   __minheap_obj_size(_heap), _func, _args)

I changed it to map to its "eqaware" counterpart like this and the
regression does not happen again.

+#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)   \
+       __min_heap_sift_down_eqaware_inline(container_of(&(_heap)->nr,
min_heap_char, nr), _pos,        \
+                                   __minheap_obj_size(_heap), _func, _args)

Do you think this correction is appropriate?

Best regards
Robert Pang

On Wed, Jun 11, 2025 at 6:55 AM Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
>
> The existing min_heap_sift_down() uses the bottom-up heapify variant,
> which reduces the number of comparisons from ~2 * log2(n) to
> ~1 * log2(n) when all elements are distinct. However, in workloads
> where the heap contains many equal elements, this bottom-up variant
> can degenerate and perform up to 2 * log2(n) comparisons, while the
> traditional top-down variant needs only O(1) comparisons in such cases.
>
> To address this, introduce min_heap_sift_down_eqaware(), a top-down
> heapify variant optimized for scenarios with many equal elements. This
> variant avoids unnecessary comparisons and swaps when elements are
> already equal or in the correct position.
>
> Cc: stable@vger.kernel.org # 6.11+
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
>  include/linux/min_heap.h | 51 ++++++++++++++++++++++++++++++++++++++++
>  lib/min_heap.c           |  7 ++++++
>  2 files changed, 58 insertions(+)
>
> diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
> index 79ddc0adbf2b..b0d603fe5379 100644
> --- a/include/linux/min_heap.h
> +++ b/include/linux/min_heap.h
> @@ -292,6 +292,52 @@ void __min_heap_sift_down_inline(min_heap_char *heap, size_t pos, size_t elem_si
>         __min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos,        \
>                                     __minheap_obj_size(_heap), _func, _args)
>
> +/*
> + * Sift the element at pos down the heap.
> + *
> + * Variants of heap functions using an equal-elements-aware sift_down.
> + * These may perform better when the heap contains many equal elements.
> + */
> +static __always_inline
> +void __min_heap_sift_down_eqaware_inline(min_heap_char * heap, size_t pos, size_t elem_size,
> +                                        const struct min_heap_callbacks *func, void *args)
> +{
> +       void *data = heap->data;
> +       void (*swp)(void *lhs, void *rhs, void *args) = func->swp;
> +       /* pre-scale counters for performance */
> +       size_t a = pos * elem_size;
> +       size_t b, c, smallest;
> +       size_t n = heap->nr * elem_size;
> +
> +       if (!swp)
> +               swp = select_swap_func(data, elem_size);
> +
> +       for (;;) {
> +               b = 2 * a + elem_size;
> +               c = b + elem_size;
> +               smallest = a;
> +
> +               if (b >= n)
> +                       break;
> +
> +               if (func->less(data + b, data + smallest, args))
> +                       smallest = b;
> +
> +               if (c < n && func->less(data + c, data + smallest, args))
> +                       smallest = c;
> +
> +               if (smallest == a)
> +                       break;
> +
> +               do_swap(data + a, data + smallest, elem_size, swp, args);
> +               a = smallest;
> +       }
> +}
> +
> +#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)   \
> +       __min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos,        \
> +                                   __minheap_obj_size(_heap), _func, _args)
> +
>  /* Sift up ith element from the heap, O(log2(nr)). */
>  static __always_inline
>  void __min_heap_sift_up_inline(min_heap_char *heap, size_t elem_size, size_t idx,
> @@ -433,6 +479,8 @@ void *__min_heap_peek(struct min_heap_char *heap);
>  bool __min_heap_full(min_heap_char *heap);
>  void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
>                           const struct min_heap_callbacks *func, void *args);
> +void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
> +                                 const struct min_heap_callbacks *func, void *args);
>  void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
>                         const struct min_heap_callbacks *func, void *args);
>  void __min_heapify_all(min_heap_char *heap, size_t elem_size,
> @@ -455,6 +503,9 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
>  #define min_heap_sift_down(_heap, _pos, _func, _args)  \
>         __min_heap_sift_down(container_of(&(_heap)->nr, min_heap_char, nr), _pos,       \
>                              __minheap_obj_size(_heap), _func, _args)
> +#define min_heap_sift_down_eqaware(_heap, _pos, _func, _args)  \
> +       __min_heap_sift_down_eqaware(container_of(&(_heap)->nr, min_heap_char, nr), _pos,       \
> +                            __minheap_obj_size(_heap), _func, _args)
>  #define min_heap_sift_up(_heap, _idx, _func, _args)    \
>         __min_heap_sift_up(container_of(&(_heap)->nr, min_heap_char, nr),       \
>                            __minheap_obj_size(_heap), _idx, _func, _args)
> diff --git a/lib/min_heap.c b/lib/min_heap.c
> index 96f01a4c5fb6..2225f40d0d7a 100644
> --- a/lib/min_heap.c
> +++ b/lib/min_heap.c
> @@ -27,6 +27,13 @@ void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
>  }
>  EXPORT_SYMBOL(__min_heap_sift_down);
>
> +void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
> +                                 const struct min_heap_callbacks *func, void *args)
> +{
> +       __min_heap_sift_down_eqaware_inline(heap, pos, elem_size, func, args);
> +}
> +EXPORT_SYMBOL(__min_heap_sift_down_eqaware);
> +
>  void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
>                         const struct min_heap_callbacks *func, void *args)
>  {
> --
> 2.34.1
>

Re: [PATCH 1/8] lib min_heap: Add equal-elements-aware sift_down variant

[Kuan-Wei Chiu]

On Thu, Jun 12, 2025 at 10:00:14PM +0900, Robert Pang wrote:
> Hi Kuan-Wei
> 
> Thanks for this patch series to address the bcache latency regression.
> I tested it but results show regression still remains. Upon review of
> the patch changes, I notice that the min_heap_sift_down_eqaware_inline
> #define macro in this patch may have been mapped incorrectly:
> 
> +#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)   \
> +       __min_heap_sift_down_inline(container_of(&(_heap)->nr,
> min_heap_char, nr), _pos,        \
> +                                   __minheap_obj_size(_heap), _func, _args)
> 
> I changed it to map to its "eqaware" counterpart like this and the
> regression does not happen again.
> 
> +#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)   \
> +       __min_heap_sift_down_eqaware_inline(container_of(&(_heap)->nr,
> min_heap_char, nr), _pos,        \
> +                                   __minheap_obj_size(_heap), _func, _args)
> 
> Do you think this correction is appropriate?
> 
That's definitely my mistake.
Thanks for testing and pointing it out.
I'll fix the typo in the next version.

Regards,
Kuan-Wei

> Best regards
> Robert Pang
> 
> On Wed, Jun 11, 2025 at 6:55 AM Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
> >
> > The existing min_heap_sift_down() uses the bottom-up heapify variant,
> > which reduces the number of comparisons from ~2 * log2(n) to
> > ~1 * log2(n) when all elements are distinct. However, in workloads
> > where the heap contains many equal elements, this bottom-up variant
> > can degenerate and perform up to 2 * log2(n) comparisons, while the
> > traditional top-down variant needs only O(1) comparisons in such cases.
> >
> > To address this, introduce min_heap_sift_down_eqaware(), a top-down
> > heapify variant optimized for scenarios with many equal elements. This
> > variant avoids unnecessary comparisons and swaps when elements are
> > already equal or in the correct position.
> >
> > Cc: stable@vger.kernel.org # 6.11+
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> > ---
> >  include/linux/min_heap.h | 51 ++++++++++++++++++++++++++++++++++++++++
> >  lib/min_heap.c           |  7 ++++++
> >  2 files changed, 58 insertions(+)
> >
> > diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
> > index 79ddc0adbf2b..b0d603fe5379 100644
> > --- a/include/linux/min_heap.h
> > +++ b/include/linux/min_heap.h
> > @@ -292,6 +292,52 @@ void __min_heap_sift_down_inline(min_heap_char *heap, size_t pos, size_t elem_si
> >         __min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos,        \
> >                                     __minheap_obj_size(_heap), _func, _args)
> >
> > +/*
> > + * Sift the element at pos down the heap.
> > + *
> > + * Variants of heap functions using an equal-elements-aware sift_down.
> > + * These may perform better when the heap contains many equal elements.
> > + */
> > +static __always_inline
> > +void __min_heap_sift_down_eqaware_inline(min_heap_char * heap, size_t pos, size_t elem_size,
> > +                                        const struct min_heap_callbacks *func, void *args)
> > +{
> > +       void *data = heap->data;
> > +       void (*swp)(void *lhs, void *rhs, void *args) = func->swp;
> > +       /* pre-scale counters for performance */
> > +       size_t a = pos * elem_size;
> > +       size_t b, c, smallest;
> > +       size_t n = heap->nr * elem_size;
> > +
> > +       if (!swp)
> > +               swp = select_swap_func(data, elem_size);
> > +
> > +       for (;;) {
> > +               b = 2 * a + elem_size;
> > +               c = b + elem_size;
> > +               smallest = a;
> > +
> > +               if (b >= n)
> > +                       break;
> > +
> > +               if (func->less(data + b, data + smallest, args))
> > +                       smallest = b;
> > +
> > +               if (c < n && func->less(data + c, data + smallest, args))
> > +                       smallest = c;
> > +
> > +               if (smallest == a)
> > +                       break;
> > +
> > +               do_swap(data + a, data + smallest, elem_size, swp, args);
> > +               a = smallest;
> > +       }
> > +}
> > +
> > +#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args)   \
> > +       __min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos,        \
> > +                                   __minheap_obj_size(_heap), _func, _args)
> > +
> >  /* Sift up ith element from the heap, O(log2(nr)). */
> >  static __always_inline
> >  void __min_heap_sift_up_inline(min_heap_char *heap, size_t elem_size, size_t idx,
> > @@ -433,6 +479,8 @@ void *__min_heap_peek(struct min_heap_char *heap);
> >  bool __min_heap_full(min_heap_char *heap);
> >  void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
> >                           const struct min_heap_callbacks *func, void *args);
> > +void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
> > +                                 const struct min_heap_callbacks *func, void *args);
> >  void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
> >                         const struct min_heap_callbacks *func, void *args);
> >  void __min_heapify_all(min_heap_char *heap, size_t elem_size,
> > @@ -455,6 +503,9 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
> >  #define min_heap_sift_down(_heap, _pos, _func, _args)  \
> >         __min_heap_sift_down(container_of(&(_heap)->nr, min_heap_char, nr), _pos,       \
> >                              __minheap_obj_size(_heap), _func, _args)
> > +#define min_heap_sift_down_eqaware(_heap, _pos, _func, _args)  \
> > +       __min_heap_sift_down_eqaware(container_of(&(_heap)->nr, min_heap_char, nr), _pos,       \
> > +                            __minheap_obj_size(_heap), _func, _args)
> >  #define min_heap_sift_up(_heap, _idx, _func, _args)    \
> >         __min_heap_sift_up(container_of(&(_heap)->nr, min_heap_char, nr),       \
> >                            __minheap_obj_size(_heap), _idx, _func, _args)
> > diff --git a/lib/min_heap.c b/lib/min_heap.c
> > index 96f01a4c5fb6..2225f40d0d7a 100644
> > --- a/lib/min_heap.c
> > +++ b/lib/min_heap.c
> > @@ -27,6 +27,13 @@ void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
> >  }
> >  EXPORT_SYMBOL(__min_heap_sift_down);
> >
> > +void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
> > +                                 const struct min_heap_callbacks *func, void *args)
> > +{
> > +       __min_heap_sift_down_eqaware_inline(heap, pos, elem_size, func, args);
> > +}
> > +EXPORT_SYMBOL(__min_heap_sift_down_eqaware);
> > +
> >  void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
> >                         const struct min_heap_callbacks *func, void *args)
> >  {
> > --
> > 2.34.1
> >

[PATCH 2/8] lib min_heap: Add typedef for sift_down function pointer

[Kuan-Wei Chiu]

Several min-heap operations such as min_heapify_all(), min_heap_pop(),
min_heap_pop_push(), and min_heap_del() use min_heap_sift_down()
internally. With the addition of the equal-elements-aware variant
min_heap_sift_down_eqaware(), these functions now need to choose
between multiple sift_down implementations.

Introduce a siftdown_fn_t typedef to represent the function pointer
type for sift_down routines. This avoids repeating verbose function
pointer declarations and simplifies code that dynamically selects the
appropriate implementation based on heap characteristics.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/min_heap.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
index b0d603fe5379..4cd8fd9db259 100644
--- a/include/linux/min_heap.h
+++ b/include/linux/min_heap.h
@@ -49,6 +49,9 @@ struct min_heap_callbacks {
 	void (*swp)(void *lhs, void *rhs, void *args);
 };
 
+typedef void (*siftdown_fn_t)(min_heap_char *heap, size_t pos, size_t elem_size,
+			    const struct min_heap_callbacks *func, void *args);
+
 /**
  * is_aligned - is this pointer & size okay for word-wide copying?
  * @base: pointer to data
-- 
2.34.1

[PATCH 3/8] lib min_heap: add eqaware variant of min_heapify_all()

[Kuan-Wei Chiu]

Introduce min_heapify_all_eqaware() as a variant of min_heapify_all()
that uses the equality-aware version of sift_down, which is implemented
in a top-down manner.

This top-down sift_down reduces the number of comparisons from
O(log2(n)) to O(1) in cases where many elements have equal priority. It
enables more efficient heap construction when the heap contains a large
number of equal elements.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/min_heap.h | 19 ++++++++++++++-----
 lib/min_heap.c           |  4 ++--
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
index 4cd8fd9db259..c2f6e1450505 100644
--- a/include/linux/min_heap.h
+++ b/include/linux/min_heap.h
@@ -371,17 +371,23 @@ void __min_heap_sift_up_inline(min_heap_char *heap, size_t elem_size, size_t idx
 /* Floyd's approach to heapification that is O(nr). */
 static __always_inline
 void __min_heapify_all_inline(min_heap_char *heap, size_t elem_size,
-			      const struct min_heap_callbacks *func, void *args)
+			      const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
 	ssize_t i;
+	siftdown_fn_t sift_down = eqaware ? __min_heap_sift_down_eqaware_inline :
+					    __min_heap_sift_down_inline;
 
 	for (i = heap->nr / 2 - 1; i >= 0; i--)
-		__min_heap_sift_down_inline(heap, i, elem_size, func, args);
+		sift_down(heap, i, elem_size, func, args);
 }
 
 #define min_heapify_all_inline(_heap, _func, _args)	\
 	__min_heapify_all_inline(container_of(&(_heap)->nr, min_heap_char, nr),	\
-				 __minheap_obj_size(_heap), _func, _args)
+				 __minheap_obj_size(_heap), _func, _args, false)
+
+#define min_heapify_all_eqaware_inline(_heap, _func, _args)	\
+	__min_heapify_all_inline(container_of(&(_heap)->nr, min_heap_char, nr),	\
+				 __minheap_obj_size(_heap), _func, _args, true)
 
 /* Remove minimum element from the heap, O(log2(nr)). */
 static __always_inline
@@ -487,7 +493,7 @@ void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_s
 void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
 			const struct min_heap_callbacks *func, void *args);
 void __min_heapify_all(min_heap_char *heap, size_t elem_size,
-		       const struct min_heap_callbacks *func, void *args);
+		       const struct min_heap_callbacks *func, void *args, bool eqaware);
 bool __min_heap_pop(min_heap_char *heap, size_t elem_size,
 		    const struct min_heap_callbacks *func, void *args);
 void __min_heap_pop_push(min_heap_char *heap, const void *element, size_t elem_size,
@@ -514,7 +520,10 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
 			   __minheap_obj_size(_heap), _idx, _func, _args)
 #define min_heapify_all(_heap, _func, _args)	\
 	__min_heapify_all(container_of(&(_heap)->nr, min_heap_char, nr),	\
-			  __minheap_obj_size(_heap), _func, _args)
+			  __minheap_obj_size(_heap), _func, _args, false)
+#define min_heapify_all_eqaware(_heap, _func, _args)	\
+	__min_heapify_all(container_of(&(_heap)->nr, min_heap_char, nr),	\
+			  __minheap_obj_size(_heap), _func, _args, true)
 #define min_heap_pop(_heap, _func, _args)	\
 	__min_heap_pop(container_of(&(_heap)->nr, min_heap_char, nr),	\
 		       __minheap_obj_size(_heap), _func, _args)
diff --git a/lib/min_heap.c b/lib/min_heap.c
index 2225f40d0d7a..a422cfaff196 100644
--- a/lib/min_heap.c
+++ b/lib/min_heap.c
@@ -42,9 +42,9 @@ void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
 EXPORT_SYMBOL(__min_heap_sift_up);
 
 void __min_heapify_all(min_heap_char *heap, size_t elem_size,
-		       const struct min_heap_callbacks *func, void *args)
+		       const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
-	__min_heapify_all_inline(heap, elem_size, func, args);
+	__min_heapify_all_inline(heap, elem_size, func, args, eqaware);
 }
 EXPORT_SYMBOL(__min_heapify_all);
 
-- 
2.34.1

[PATCH 4/8] lib min_heap: add eqaware variant of min_heap_pop()

[Kuan-Wei Chiu]

Introduce min_heap_pop_eqaware() as a variant of min_heap_pop() that
uses the equality-aware version of sift_down, which is implemented in a
top-down manner.

This top-down sift_down reduces the number of comparisons from
O(log2(n)) to O(1) in cases where many elements have equal priority. It
enables more efficient heap construction when the heap contains a large
number of equal elements.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/min_heap.h | 19 ++++++++++++++-----
 lib/min_heap.c           |  4 ++--
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
index c2f6e1450505..6c45d617b027 100644
--- a/include/linux/min_heap.h
+++ b/include/linux/min_heap.h
@@ -392,9 +392,11 @@ void __min_heapify_all_inline(min_heap_char *heap, size_t elem_size,
 /* Remove minimum element from the heap, O(log2(nr)). */
 static __always_inline
 bool __min_heap_pop_inline(min_heap_char *heap, size_t elem_size,
-			   const struct min_heap_callbacks *func, void *args)
+			   const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
 	void *data = heap->data;
+	siftdown_fn_t sift_down = eqaware ? __min_heap_sift_down_eqaware_inline :
+					    __min_heap_sift_down_inline;
 
 	if (WARN_ONCE(heap->nr <= 0, "Popping an empty heap"))
 		return false;
@@ -402,14 +404,18 @@ bool __min_heap_pop_inline(min_heap_char *heap, size_t elem_size,
 	/* Place last element at the root (position 0) and then sift down. */
 	heap->nr--;
 	memcpy(data, data + (heap->nr * elem_size), elem_size);
-	__min_heap_sift_down_inline(heap, 0, elem_size, func, args);
+	sift_down(heap, 0, elem_size, func, args);
 
 	return true;
 }
 
 #define min_heap_pop_inline(_heap, _func, _args)	\
 	__min_heap_pop_inline(container_of(&(_heap)->nr, min_heap_char, nr),	\
-			      __minheap_obj_size(_heap), _func, _args)
+			      __minheap_obj_size(_heap), _func, _args, false)
+
+#define min_heap_pop_eqaware_inline(_heap, _func, _args)	\
+	__min_heap_pop_inline(container_of(&(_heap)->nr, min_heap_char, nr),	\
+			      __minheap_obj_size(_heap), _func, _args, true)
 
 /*
  * Remove the minimum element and then push the given element. The
@@ -495,7 +501,7 @@ void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
 void __min_heapify_all(min_heap_char *heap, size_t elem_size,
 		       const struct min_heap_callbacks *func, void *args, bool eqaware);
 bool __min_heap_pop(min_heap_char *heap, size_t elem_size,
-		    const struct min_heap_callbacks *func, void *args);
+		    const struct min_heap_callbacks *func, void *args, bool eqaware);
 void __min_heap_pop_push(min_heap_char *heap, const void *element, size_t elem_size,
 			 const struct min_heap_callbacks *func, void *args);
 bool __min_heap_push(min_heap_char *heap, const void *element, size_t elem_size,
@@ -526,7 +532,10 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
 			  __minheap_obj_size(_heap), _func, _args, true)
 #define min_heap_pop(_heap, _func, _args)	\
 	__min_heap_pop(container_of(&(_heap)->nr, min_heap_char, nr),	\
-		       __minheap_obj_size(_heap), _func, _args)
+		       __minheap_obj_size(_heap), _func, _args, false)
+#define min_heap_pop_eqaware(_heap, _func, _args)	\
+	__min_heap_pop(container_of(&(_heap)->nr, min_heap_char, nr),	\
+		       __minheap_obj_size(_heap), _func, _args, true)
 #define min_heap_pop_push(_heap, _element, _func, _args)	\
 	__min_heap_pop_push(container_of(&(_heap)->nr, min_heap_char, nr), _element,	\
 			    __minheap_obj_size(_heap), _func, _args)
diff --git a/lib/min_heap.c b/lib/min_heap.c
index a422cfaff196..dae3ed39421a 100644
--- a/lib/min_heap.c
+++ b/lib/min_heap.c
@@ -49,9 +49,9 @@ void __min_heapify_all(min_heap_char *heap, size_t elem_size,
 EXPORT_SYMBOL(__min_heapify_all);
 
 bool __min_heap_pop(min_heap_char *heap, size_t elem_size,
-		    const struct min_heap_callbacks *func, void *args)
+		    const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
-	return __min_heap_pop_inline(heap, elem_size, func, args);
+	return __min_heap_pop_inline(heap, elem_size, func, args, eqaware);
 }
 EXPORT_SYMBOL(__min_heap_pop);
 
-- 
2.34.1

[PATCH 5/8] lib min_heap: add eqaware variant of min_heap_pop_push()

[Kuan-Wei Chiu]

Introduce min_heap_pop_push_eqaware() as a variant of
min_heap_pop_push() that uses the equality-aware version of sift_down,
which is implemented in a top-down manner.

This top-down sift_down reduces the number of comparisons from
O(log2(n)) to O(1) in cases where many elements have equal priority. It
enables more efficient heap construction when the heap contains a large
number of equal elements.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/min_heap.h | 20 +++++++++++++++-----
 lib/min_heap.c           |  4 ++--
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
index 6c45d617b027..d7bf8dd0f6b1 100644
--- a/include/linux/min_heap.h
+++ b/include/linux/min_heap.h
@@ -424,15 +424,22 @@ bool __min_heap_pop_inline(min_heap_char *heap, size_t elem_size,
  */
 static __always_inline
 void __min_heap_pop_push_inline(min_heap_char *heap, const void *element, size_t elem_size,
-				const struct min_heap_callbacks *func, void *args)
+				const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
+	siftdown_fn_t sift_down = eqaware ? __min_heap_sift_down_eqaware_inline :
+					    __min_heap_sift_down_inline;
+
 	memcpy(heap->data, element, elem_size);
-	__min_heap_sift_down_inline(heap, 0, elem_size, func, args);
+	sift_down(heap, 0, elem_size, func, args);
 }
 
 #define min_heap_pop_push_inline(_heap, _element, _func, _args)	\
 	__min_heap_pop_push_inline(container_of(&(_heap)->nr, min_heap_char, nr), _element,	\
-				   __minheap_obj_size(_heap), _func, _args)
+				   __minheap_obj_size(_heap), _func, _args, false)
+
+#define min_heap_pop_push_eqaware_inline(_heap, _element, _func, _args)	\
+	__min_heap_pop_push_inline(container_of(&(_heap)->nr, min_heap_char, nr), _element,	\
+				   __minheap_obj_size(_heap), _func, _args, true)
 
 /* Push an element on to the heap, O(log2(nr)). */
 static __always_inline
@@ -503,7 +510,7 @@ void __min_heapify_all(min_heap_char *heap, size_t elem_size,
 bool __min_heap_pop(min_heap_char *heap, size_t elem_size,
 		    const struct min_heap_callbacks *func, void *args, bool eqaware);
 void __min_heap_pop_push(min_heap_char *heap, const void *element, size_t elem_size,
-			 const struct min_heap_callbacks *func, void *args);
+			 const struct min_heap_callbacks *func, void *args, bool eqaware);
 bool __min_heap_push(min_heap_char *heap, const void *element, size_t elem_size,
 		     const struct min_heap_callbacks *func, void *args);
 bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
@@ -538,7 +545,10 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
 		       __minheap_obj_size(_heap), _func, _args, true)
 #define min_heap_pop_push(_heap, _element, _func, _args)	\
 	__min_heap_pop_push(container_of(&(_heap)->nr, min_heap_char, nr), _element,	\
-			    __minheap_obj_size(_heap), _func, _args)
+			    __minheap_obj_size(_heap), _func, _args, false)
+#define min_heap_pop_push_eqaware(_heap, _element, _func, _args)	\
+	__min_heap_pop_push(container_of(&(_heap)->nr, min_heap_char, nr), _element,	\
+			    __minheap_obj_size(_heap), _func, _args, true)
 #define min_heap_push(_heap, _element, _func, _args)	\
 	__min_heap_push(container_of(&(_heap)->nr, min_heap_char, nr), _element,	\
 			__minheap_obj_size(_heap), _func, _args)
diff --git a/lib/min_heap.c b/lib/min_heap.c
index dae3ed39421a..a69d8b80d443 100644
--- a/lib/min_heap.c
+++ b/lib/min_heap.c
@@ -56,9 +56,9 @@ bool __min_heap_pop(min_heap_char *heap, size_t elem_size,
 EXPORT_SYMBOL(__min_heap_pop);
 
 void __min_heap_pop_push(min_heap_char *heap, const void *element, size_t elem_size,
-			 const struct min_heap_callbacks *func, void *args)
+			 const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
-	__min_heap_pop_push_inline(heap, element, elem_size, func, args);
+	__min_heap_pop_push_inline(heap, element, elem_size, func, args, eqaware);
 }
 EXPORT_SYMBOL(__min_heap_pop_push);
 
-- 
2.34.1

[PATCH 6/8] lib min_heap: add eqaware variant of min_heap_del()

[Kuan-Wei Chiu]

Introduce min_heap_del_eqaware() as a variant of min_heap_del() that
uses the equality-aware version of sift_down, which is implemented in a
top-down manner.

This top-down sift_down reduces the number of comparisons from
O(log2(n)) to O(1) in cases where many elements have equal priority. It
enables more efficient heap construction when the heap contains a large
number of equal elements.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/min_heap.h | 19 ++++++++++++++-----
 lib/min_heap.c           |  4 ++--
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
index d7bf8dd0f6b1..f34df8dd2e17 100644
--- a/include/linux/min_heap.h
+++ b/include/linux/min_heap.h
@@ -470,10 +470,12 @@ bool __min_heap_push_inline(min_heap_char *heap, const void *element, size_t ele
 /* Remove ith element from the heap, O(log2(nr)). */
 static __always_inline
 bool __min_heap_del_inline(min_heap_char *heap, size_t elem_size, size_t idx,
-			   const struct min_heap_callbacks *func, void *args)
+			   const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
 	void *data = heap->data;
 	void (*swp)(void *lhs, void *rhs, void *args) = func->swp;
+	siftdown_fn_t sift_down = eqaware ? __min_heap_sift_down_eqaware_inline :
+					    __min_heap_sift_down_inline;
 
 	if (WARN_ONCE(heap->nr <= 0, "Popping an empty heap"))
 		return false;
@@ -487,14 +489,18 @@ bool __min_heap_del_inline(min_heap_char *heap, size_t elem_size, size_t idx,
 		return true;
 	do_swap(data + (idx * elem_size), data + (heap->nr * elem_size), elem_size, swp, args);
 	__min_heap_sift_up_inline(heap, elem_size, idx, func, args);
-	__min_heap_sift_down_inline(heap, idx, elem_size, func, args);
+	sift_down(heap, idx, elem_size, func, args);
 
 	return true;
 }
 
 #define min_heap_del_inline(_heap, _idx, _func, _args)	\
 	__min_heap_del_inline(container_of(&(_heap)->nr, min_heap_char, nr),	\
-			      __minheap_obj_size(_heap), _idx, _func, _args)
+			      __minheap_obj_size(_heap), _idx, _func, _args, false)
+
+#define min_heap_del_eqaware_inline(_heap, _idx, _func, _args)	\
+	__min_heap_del_inline(container_of(&(_heap)->nr, min_heap_char, nr),	\
+			      __minheap_obj_size(_heap), _idx, _func, _args, true)
 
 void __min_heap_init(min_heap_char *heap, void *data, size_t size);
 void *__min_heap_peek(struct min_heap_char *heap);
@@ -514,7 +520,7 @@ void __min_heap_pop_push(min_heap_char *heap, const void *element, size_t elem_s
 bool __min_heap_push(min_heap_char *heap, const void *element, size_t elem_size,
 		     const struct min_heap_callbacks *func, void *args);
 bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
-		    const struct min_heap_callbacks *func, void *args);
+		    const struct min_heap_callbacks *func, void *args, bool eqaware);
 
 #define min_heap_init(_heap, _data, _size)	\
 	__min_heap_init(container_of(&(_heap)->nr, min_heap_char, nr), _data, _size)
@@ -554,6 +560,9 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
 			__minheap_obj_size(_heap), _func, _args)
 #define min_heap_del(_heap, _idx, _func, _args)	\
 	__min_heap_del(container_of(&(_heap)->nr, min_heap_char, nr),	\
-		       __minheap_obj_size(_heap), _idx, _func, _args)
+		       __minheap_obj_size(_heap), _idx, _func, _args, false)
+#define min_heap_del_eqaware(_heap, _idx, _func, _args)	\
+	__min_heap_del(container_of(&(_heap)->nr, min_heap_char, nr),	\
+		       __minheap_obj_size(_heap), _idx, _func, _args, true)
 
 #endif /* _LINUX_MIN_HEAP_H */
diff --git a/lib/min_heap.c b/lib/min_heap.c
index a69d8b80d443..50f224f174d5 100644
--- a/lib/min_heap.c
+++ b/lib/min_heap.c
@@ -70,8 +70,8 @@ bool __min_heap_push(min_heap_char *heap, const void *element, size_t elem_size,
 EXPORT_SYMBOL(__min_heap_push);
 
 bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
-		    const struct min_heap_callbacks *func, void *args)
+		    const struct min_heap_callbacks *func, void *args, bool eqaware)
 {
-	return __min_heap_del_inline(heap, elem_size, idx, func, args);
+	return __min_heap_del_inline(heap, elem_size, idx, func, args, eqaware);
 }
 EXPORT_SYMBOL(__min_heap_del);
-- 
2.34.1

[PATCH 7/8] Documentation/core-api: min_heap: Document _eqaware variants of min-heap APIs

[Kuan-Wei Chiu]

Add documentation for equality-aware variants of min-heap maintenance
functions, which use a top-down sift_down strategy. These variants,
suffixed with _eqaware, reduce the number of comparisons to O(1) in
workloads with many elements of equal priority and can be used as
drop-in replacements for their standard counterparts.

Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 Documentation/core-api/min_heap.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/core-api/min_heap.rst b/Documentation/core-api/min_heap.rst
index 9f57766581df..012c82038b46 100644
--- a/Documentation/core-api/min_heap.rst
+++ b/Documentation/core-api/min_heap.rst
@@ -30,6 +30,26 @@ more expensive. As with the non-inline versions, it is important to use the
 macro wrappers for inline functions instead of directly calling the functions
 themselves.
 
+Equality-Aware Heap Maintenance
+-------------------------------
+
+In some workloads, a large number of elements in the heap may be equal under
+the user-defined comparison function. For such cases, the standard
+``min_heap_sift_down()`` implementation, which uses the bottom-up heapify
+strategy, can be inefficient. While bottom-up heapify reduces the number of
+comparisons by approximately 50% for randomly ordered data, it may perform up
+to :math:`2 \times \log_2(n)` comparisons in the presence of many equal
+elements.
+
+To address this, equality-aware versions of heap maintenance APIs are provided.
+These versions use the traditional top-down heapify strategy, which performs
+better - sometimes requiring only :math:`\mathcal{O}(1)` comparisons - when
+many elements are equal.
+
+The equality-aware APIs are suffixed with ``_eqaware``, and serve as drop-in
+replacements for their standard counterparts when equal elements are expected.
+
+
 Data Structures
 ===============
 
-- 
2.34.1

[PATCH 8/8] bcache: Fix the tail IO latency regression by using equality-aware min heap API

[Kuan-Wei Chiu]

Commit 866898efbb25 ("bcache: remove heap-related macros and switch to
generic min_heap") replaced the original top-down heap macros in bcache
with the generic min heap library, which uses a bottom-up heapify
strategy. However, in scenarios like invalidate_buckets_lru() -
especially before the cache is fully populated - many buckets remain
unfilled. This causes new_bucket_prio() to frequently return zero,
leading to a high rate of equal comparisons.

Bottom-up sift_down performs up to 2 * log2(n) comparisons in such
cases, resulting in a performance regression.

Switch to the _eqaware variants of the min heap API to restore the
original top-down sift_down behavior, which requires only O(1)
comparisons when many elements are equal.

Also use the inline versions of the heap functions to avoid performance
degradation introduced by commit 92a8b224b833 ("lib/min_heap: introduce
non-inline versions of min heap API functions"), as
invalidate_buckets_lru() is on a performance-critical hot path.

Fixes: 866898efbb25 ("bcache: remove heap-related macros and switch to generic min_heap")
Fixes: 92a8b224b833 ("lib/min_heap: introduce non-inline versions of min heap API functions")
Reported-by: Robert Pang <robertpang@google.com>
Closes: https://lore.kernel.org/linux-bcache/CAJhEC06F_AtrPgw2-7CvCqZgeStgCtitbD-ryuPpXQA-JG5XXw@mail.gmail.com
Cc: stable@vger.kernel.org # 6.11+
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 drivers/md/bcache/alloc.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index 8998e61efa40..625c5c4eb962 100644
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -207,15 +207,16 @@ static void invalidate_buckets_lru(struct cache *ca)
 		if (!bch_can_invalidate_bucket(ca, b))
 			continue;
 
-		if (!min_heap_full(&ca->heap))
-			min_heap_push(&ca->heap, &b, &bucket_max_cmp_callback, ca);
-		else if (!new_bucket_max_cmp(&b, min_heap_peek(&ca->heap), ca)) {
+		if (!min_heap_full_inline(&ca->heap))
+			min_heap_push_inline(&ca->heap, &b, &bucket_max_cmp_callback, ca);
+		else if (!new_bucket_max_cmp(&b, min_heap_peek_inline(&ca->heap), ca)) {
 			ca->heap.data[0] = b;
-			min_heap_sift_down(&ca->heap, 0, &bucket_max_cmp_callback, ca);
+			min_heap_sift_down_eqaware_inline(&ca->heap, 0, &bucket_max_cmp_callback,
+							  ca);
 		}
 	}
 
-	min_heapify_all(&ca->heap, &bucket_min_cmp_callback, ca);
+	min_heapify_all_eqaware_inline(&ca->heap, &bucket_min_cmp_callback, ca);
 
 	while (!fifo_full(&ca->free_inc)) {
 		if (!ca->heap.nr) {
@@ -227,8 +228,8 @@ static void invalidate_buckets_lru(struct cache *ca)
 			wake_up_gc(ca->set);
 			return;
 		}
-		b = min_heap_peek(&ca->heap)[0];
-		min_heap_pop(&ca->heap, &bucket_min_cmp_callback, ca);
+		b = min_heap_peek_inline(&ca->heap)[0];
+		min_heap_pop_eqaware_inline(&ca->heap, &bucket_min_cmp_callback, ca);
 
 		bch_invalidate_one_bucket(ca, b);
 	}
-- 
2.34.1

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Andrew Morton]

On Wed, 11 Jun 2025 05:55:08 +0800 Kuan-Wei Chiu <visitorckw@gmail.com> wrote:

> This patch series introduces equality-aware variants of the min heap
> API that use a top-down heapify strategy to improve performance when
> many elements are equal under the comparison function. It also updates
> the documentation accordingly and modifies bcache to use the new APIs
> to fix a performance regression caused by the switch to the generic min
> heap library.
> 
> In particular, invalidate_buckets_lru() in bcache suffered from
> increased comparison overhead due to the bottom-up strategy introduced
> in commit 866898efbb25 ("bcache: remove heap-related macros and switch
> to generic min_heap"). The regression is addressed by switching to the
> equality-aware variants and using the inline versions to avoid function
> call overhead in this hot path.
> 
> Cc: stable@vger.kernel.org

To justify a -stable backport this performance regression would need to
have a pretty significant impact upon real-world userspace.  Especially
as the patchset is large.

Unfortunately the changelog provides no indication of the magnitude of
the userspace impact.   Please tell us this, in detail.

Also, if we are to address this regression in -stable kernels then
reverting 866898efbb25 is an obvious way - it is far far safer.  So
please also tell us why the proposed patchset is a better way for us to
go.

(Also, each patch should have a fixes:866898efbb25 to help direct the
backporting efforts)


I'll add the patches to mm.git to get you some testing but from what
I'm presently seeing the -stable backporting would be unwise.

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Andrew Morton]

On Wed, 11 Jun 2025 18:48:17 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> Unfortunately the changelog provides no indication of the magnitude of
> the userspace impact.   Please tell us this, in detail.

OK I found details in the 8th patch's Closes: link.  That was tricky ;)

It's impressively detailed but I'm still struggling to understand how
much pain this regression causes users in real life.  So some sort of
reader-friendly executive summary would be great, please.

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Kuan-Wei Chiu]

Hi Andrew,

On Wed, Jun 11, 2025 at 06:48:17PM -0700, Andrew Morton wrote:
> On Wed, 11 Jun 2025 05:55:08 +0800 Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
> 
> > This patch series introduces equality-aware variants of the min heap
> > API that use a top-down heapify strategy to improve performance when
> > many elements are equal under the comparison function. It also updates
> > the documentation accordingly and modifies bcache to use the new APIs
> > to fix a performance regression caused by the switch to the generic min
> > heap library.
> > 
> > In particular, invalidate_buckets_lru() in bcache suffered from
> > increased comparison overhead due to the bottom-up strategy introduced
> > in commit 866898efbb25 ("bcache: remove heap-related macros and switch
> > to generic min_heap"). The regression is addressed by switching to the
> > equality-aware variants and using the inline versions to avoid function
> > call overhead in this hot path.
> > 
> > Cc: stable@vger.kernel.org
> 
> To justify a -stable backport this performance regression would need to
> have a pretty significant impact upon real-world userspace.  Especially
> as the patchset is large.
> 
> Unfortunately the changelog provides no indication of the magnitude of
> the userspace impact.   Please tell us this, in detail.
> 
I'll work with Robert to provide a more detailed explanation of the
real-world impact on userspace.

> Also, if we are to address this regression in -stable kernels then
> reverting 866898efbb25 is an obvious way - it is far far safer.  So
> please also tell us why the proposed patchset is a better way for us to
> go.
> 
I agree that reverting 866898efbb25 is a much safer and smaller change
for backporting. In fact, I previously raised the discussion of whether
we should revert the commit or instead introduce an equality-aware API
and use it. The bcache maintainer preferred the latter, and I also
believe that it is a more forward-looking approach. Given that bcache
has run into this issue, it's likely that other users with similar use
cases may encounter it as well. We wouldn't want those users to
continue relying on the current default heapify behavior. So, although
reverting may be more suitable for stable in isolation, adding an
equality-aware API could better serve a broader set of use cases going
forward.

> (Also, each patch should have a fixes:866898efbb25 to help direct the
> backporting efforts)
> 
Ack. Will do.

> 
> I'll add the patches to mm.git to get you some testing but from what
> I'm presently seeing the -stable backporting would be unwise.

Thanks!

Regards,
Kuan-Wei

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Robert Pang]

Hi Andrew

Bcache is designed to boost the I/O performance of slower storage
(HDDs, network-attached storage) by leveraging fast SSDs as a block
cache. This functionality is critical in significantly reducing I/O
latency. Therefore, any notable increase in bcache's latency severely
diminishes its value. For instance, our tests show a P100 (max)
latency spike from 600 ms to 2.4 seconds every 5 minutes due to this
regression. In real-world environments, this  increase will cause
frequent timeouts and stalls in end-user applications that rely on
bcache's latency improvements, highlighting the urgent need to address
this issue.

Best regards
Robert Pang

On Fri, Jun 13, 2025 at 3:16 PM Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
>
> Hi Andrew,
>
> On Wed, Jun 11, 2025 at 06:48:17PM -0700, Andrew Morton wrote:
> > On Wed, 11 Jun 2025 05:55:08 +0800 Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
> >
> > > This patch series introduces equality-aware variants of the min heap
> > > API that use a top-down heapify strategy to improve performance when
> > > many elements are equal under the comparison function. It also updates
> > > the documentation accordingly and modifies bcache to use the new APIs
> > > to fix a performance regression caused by the switch to the generic min
> > > heap library.
> > >
> > > In particular, invalidate_buckets_lru() in bcache suffered from
> > > increased comparison overhead due to the bottom-up strategy introduced
> > > in commit 866898efbb25 ("bcache: remove heap-related macros and switch
> > > to generic min_heap"). The regression is addressed by switching to the
> > > equality-aware variants and using the inline versions to avoid function
> > > call overhead in this hot path.
> > >
> > > Cc: stable@vger.kernel.org
> >
> > To justify a -stable backport this performance regression would need to
> > have a pretty significant impact upon real-world userspace.  Especially
> > as the patchset is large.
> >
> > Unfortunately the changelog provides no indication of the magnitude of
> > the userspace impact.   Please tell us this, in detail.
> >
> I'll work with Robert to provide a more detailed explanation of the
> real-world impact on userspace.
>
> > Also, if we are to address this regression in -stable kernels then
> > reverting 866898efbb25 is an obvious way - it is far far safer.  So
> > please also tell us why the proposed patchset is a better way for us to
> > go.
> >
> I agree that reverting 866898efbb25 is a much safer and smaller change
> for backporting. In fact, I previously raised the discussion of whether
> we should revert the commit or instead introduce an equality-aware API
> and use it. The bcache maintainer preferred the latter, and I also
> believe that it is a more forward-looking approach. Given that bcache
> has run into this issue, it's likely that other users with similar use
> cases may encounter it as well. We wouldn't want those users to
> continue relying on the current default heapify behavior. So, although
> reverting may be more suitable for stable in isolation, adding an
> equality-aware API could better serve a broader set of use cases going
> forward.
>
> > (Also, each patch should have a fixes:866898efbb25 to help direct the
> > backporting efforts)
> >
> Ack. Will do.
>
> >
> > I'll add the patches to mm.git to get you some testing but from what
> > I'm presently seeing the -stable backporting would be unwise.
>
> Thanks!
>
> Regards,
> Kuan-Wei

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Andrew Morton]

On Fri, 13 Jun 2025 23:26:33 +0900 Robert Pang <robertpang@google.com> wrote:

> Hi Andrew
> 
> Bcache is designed to boost the I/O performance of slower storage
> (HDDs, network-attached storage) by leveraging fast SSDs as a block
> cache. This functionality is critical in significantly reducing I/O
> latency. Therefore, any notable increase in bcache's latency severely
> diminishes its value. For instance, our tests show a P100 (max)
> latency spike from 600 ms to 2.4 seconds every 5 minutes due to this
> regression. In real-world environments, this  increase will cause
> frequent timeouts and stalls in end-user applications that rely on
> bcache's latency improvements, highlighting the urgent need to address
> this issue.

Great, thanks.  Let's please incorporate this into the v2 changelogging.

> > > Also, if we are to address this regression in -stable kernels then
> > > reverting 866898efbb25 is an obvious way - it is far far safer.  So
> > > please also tell us why the proposed patchset is a better way for us to
> > > go.
> > >
> > I agree that reverting 866898efbb25 is a much safer and smaller change
> > for backporting. In fact, I previously raised the discussion of whether
> > we should revert the commit or instead introduce an equality-aware API
> > and use it. The bcache maintainer preferred the latter, and I also
> > believe that it is a more forward-looking approach. Given that bcache
> > has run into this issue, it's likely that other users with similar use
> > cases may encounter it as well. We wouldn't want those users to
> > continue relying on the current default heapify behavior. So, although
> > reverting may be more suitable for stable in isolation, adding an
> > equality-aware API could better serve a broader set of use cases going
> > forward.

"much safer and smaller" is very desirable for backporting, please. 
After all, 866898efbb25 didn't really fix anything and reverting that
takes us back to a known-to-work implementation.

I of course have no problem making the changes in this patchset for
"going forward"!

So if agreeable, please prepare a patch which reverts 866898efbb25. 
Robert's words above are a great basis for that patch's description.

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Kuan-Wei Chiu]

Hi Andrew,

On Fri, Jun 13, 2025 at 11:04:15AM -0700, Andrew Morton wrote:
> On Fri, 13 Jun 2025 23:26:33 +0900 Robert Pang <robertpang@google.com> wrote:
> 
> > Hi Andrew
> > 
> > Bcache is designed to boost the I/O performance of slower storage
> > (HDDs, network-attached storage) by leveraging fast SSDs as a block
> > cache. This functionality is critical in significantly reducing I/O
> > latency. Therefore, any notable increase in bcache's latency severely
> > diminishes its value. For instance, our tests show a P100 (max)
> > latency spike from 600 ms to 2.4 seconds every 5 minutes due to this
> > regression. In real-world environments, this  increase will cause
> > frequent timeouts and stalls in end-user applications that rely on
> > bcache's latency improvements, highlighting the urgent need to address
> > this issue.
> 
> Great, thanks.  Let's please incorporate this into the v2 changelogging.
> 
> > > > Also, if we are to address this regression in -stable kernels then
> > > > reverting 866898efbb25 is an obvious way - it is far far safer.  So
> > > > please also tell us why the proposed patchset is a better way for us to
> > > > go.
> > > >
> > > I agree that reverting 866898efbb25 is a much safer and smaller change
> > > for backporting. In fact, I previously raised the discussion of whether
> > > we should revert the commit or instead introduce an equality-aware API
> > > and use it. The bcache maintainer preferred the latter, and I also
> > > believe that it is a more forward-looking approach. Given that bcache
> > > has run into this issue, it's likely that other users with similar use
> > > cases may encounter it as well. We wouldn't want those users to
> > > continue relying on the current default heapify behavior. So, although
> > > reverting may be more suitable for stable in isolation, adding an
> > > equality-aware API could better serve a broader set of use cases going
> > > forward.
> 
> "much safer and smaller" is very desirable for backporting, please. 
> After all, 866898efbb25 didn't really fix anything and reverting that
> takes us back to a known-to-work implementation.
> 
> I of course have no problem making the changes in this patchset for
> "going forward"!
> 
> So if agreeable, please prepare a patch which reverts 866898efbb25. 
> Robert's words above are a great basis for that patch's description.
> 
Sure, I'll prepare a revert patch to address the issue and plan to
submit it for backporting to -stable.

However, I'd like to confirm whether the following patch series
structure would be appropriate:

- Patch 1: Revert 866898efbb25 and CC it to stable
- Patch 2–8: Introduce the new equality-aware heap API
- Patch 9: Revert Patch 1 and switch bcache to use the new API

In this case, we would only backport Patch 1 to stable.

Alternatively, would you prefer we simply revert 866898efbb25 without
introducing and using the new API in the same series?

Regards,
Kuan-Wei

Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

[Andrew Morton]

On Sat, 14 Jun 2025 07:19:51 +0800 Kuan-Wei Chiu <visitorckw@gmail.com> wrote:

> Sure, I'll prepare a revert patch to address the issue and plan to
> submit it for backporting to -stable.
> 
> However, I'd like to confirm whether the following patch series
> structure would be appropriate:
> 
> - Patch 1: Revert 866898efbb25 and CC it to stable
> - Patch 2–8: Introduce the new equality-aware heap API
> - Patch 9: Revert Patch 1 and switch bcache to use the new API
> 
> In this case, we would only backport Patch 1 to stable.
> 
> Alternatively, would you prefer we simply revert 866898efbb25 without
> introducing and using the new API in the same series?

Yes, just the revert for now, please.  Let's not make that dependent on
new development.