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

[Kuan-Wei Chiu <visitorckw@gmail.com>]

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
> >