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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7482af3a165sm8173936b3a.11.2025.06.10.14.55.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Jun 2025 14:55:44 -0700 (PDT) From: Kuan-Wei Chiu To: corbet@lwn.net, colyli@kernel.org, kent.overstreet@linux.dev, akpm@linux-foundation.org, robertpang@google.com Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-bcache@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , stable@vger.kernel.org Subject: [PATCH 1/8] lib min_heap: Add equal-elements-aware sift_down variant Date: Wed, 11 Jun 2025 05:55:09 +0800 Message-Id: <20250610215516.1513296-2-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250610215516.1513296-1-visitorckw@gmail.com> References: <20250610215516.1513296-1-visitorckw@gmail.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- 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