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Howlett" , Matthew Wilcox Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Andrew Morton , "Paul E. McKenney" , Steven Rostedt , Masami Hiramatsu , Dennis Zhou , Tejun Heo , Martin Liu , David Rientjes , christian.koenig@amd.com, Shakeel Butt , Johannes Weiner , Lorenzo Stoakes , Suren Baghdasaryan , Vlastimil Babka , Christian Brauner , Wei Yang , David Hildenbrand , Miaohe Lin , Al Viro , linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, Yu Zhao , Roman Gushchin , Mateusz Guzik Subject: [RFC PATCH v3] Introduce Hierarchical Per-CPU Counters Date: Thu, 10 Apr 2025 13:51:49 -0400 Message-Id: <20250410175149.1206995-1-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.5 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: ED081180005 X-Stat-Signature: ogatwuuyx5y19w44wxee5pc7sngbiyq9 X-Rspam-User: X-HE-Tag: 1744307520-940370 X-HE-Meta: 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 clTAbNpF 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 S0QPC7Za YlJzlkLO9Q4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: * Motivation The purpose of this hierarchical split-counter scheme is to: - Minimize contention when incrementing and decrementing counters, - Provide fast access to a sum approximation, - Provide a sum approximation with an acceptable accuracy level when scaling to many-core systems. - Provide approximate and precise comparison of two counters, and between a counter and a value. It aims at fixing the per-mm RSS tracking which has become too inaccurate for OOM killer purposes on large many-core systems [1]. * Design The hierarchical per-CPU counters propagate a sum approximation through a N-way tree. When reaching the batch size, the carry is propagated through a binary tree which consists of logN(nr_cpu_ids) levels. The batch size for each level is twice the batch size of the prior level. Example propagation diagram with 8 cpus through a binary tree: Level 0: 0 1 2 3 4 5 6 7 | / | / | / | / | / | / | / | / | / | / | / | / Level 1: 0 1 2 3 | / | / | / | / | / | / Level 2: 0 1 | / | / | / Level 3: 0 For a binary tree, the maximum inaccuracy is bound by: batch_size * log2(nr_cpus) * nr_cpus which evolves with O(n*log(n)) as the number of CPUs increases. For a N-way tree, the maximum inaccuracy can be pre-calculated based on the the N-arity of each level and the batch size. Link: https://lore.kernel.org/lkml/20250331223516.7810-2-sweettea-kernel@dorminy.me/ # [1] Signed-off-by: Mathieu Desnoyers Cc: Andrew Morton Cc: "Paul E. McKenney" Cc: Steven Rostedt Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: Dennis Zhou Cc: Tejun Heo Cc: Christoph Lameter Cc: Martin Liu Cc: David Rientjes Cc: christian.koenig@amd.com Cc: Shakeel Butt Cc: Johannes Weiner Cc: Sweet Tea Dorminy Cc: Lorenzo Stoakes Cc: "Liam R . Howlett" Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Christian Brauner Cc: Wei Yang Cc: David Hildenbrand Cc: Miaohe Lin Cc: Al Viro Cc: linux-mm@kvack.org Cc: linux-trace-kernel@vger.kernel.org Cc: Yu Zhao Cc: Roman Gushchin Cc: Mateusz Guzik Cc: Matthew Wilcox --- Changes since v2: - Introduce N-way tree to reduce tree depth on larger systems. Changes since v1: - Remove percpu_counter_tree_precise_sum_unbiased from public header, make this function static, - Introduce precise and approximate comparisons between two counters, - Reorder the struct percpu_counter_tree fields, - Introduce approx_sum field, which points to the approximate sum for the percpu_counter_tree_approximate_sum() fast path. --- include/linux/percpu_counter_tree.h | 108 ++++++++ lib/Makefile | 1 + lib/percpu_counter_tree.c | 393 ++++++++++++++++++++++++++++ 3 files changed, 502 insertions(+) create mode 100644 include/linux/percpu_counter_tree.h create mode 100644 lib/percpu_counter_tree.c diff --git a/include/linux/percpu_counter_tree.h b/include/linux/percpu_counter_tree.h new file mode 100644 index 000000000000..7bdbb3ffa10c --- /dev/null +++ b/include/linux/percpu_counter_tree.h @@ -0,0 +1,108 @@ +/* SPDX-License-Identifier: GPL-2.0+ OR MIT */ +/* SPDX-FileCopyrightText: 2025 Mathieu Desnoyers */ + +#ifndef _PERCPU_COUNTER_TREE_H +#define _PERCPU_COUNTER_TREE_H + +#include +#include +#include +#include + +struct percpu_counter_tree_level_item { + atomic_t count; +} ____cacheline_aligned_in_smp; + +struct percpu_counter_tree { + /* Fast-path fields. */ + unsigned int __percpu *level0; + unsigned int level0_bit_mask; + union { + unsigned int *i; + atomic_t *a; + } approx_sum; + int bias; /* bias for counter_set */ + + /* Slow-path fields. */ + struct percpu_counter_tree_level_item *items; + unsigned int batch_size; + unsigned int inaccuracy; /* approximation imprecise within ± inaccuracy */ +}; + +int percpu_counter_tree_init(struct percpu_counter_tree *counter, unsigned int batch_size); +void percpu_counter_tree_destroy(struct percpu_counter_tree *counter); +void percpu_counter_tree_add_slowpath(struct percpu_counter_tree *counter, int inc); +int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter); +int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b); +int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v); +int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b); +int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v); +void percpu_counter_tree_set_bias(struct percpu_counter_tree *counter, int bias); +void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v); +unsigned int percpu_counter_tree_inaccuracy(struct percpu_counter_tree *counter); + +/* Fast paths */ + +static inline +int percpu_counter_tree_carry(int orig, int res, int inc, unsigned int bit_mask) +{ + if (inc < 0) { + inc = -(-inc & ~(bit_mask - 1)); + /* + * xor bit_mask: underflow. + * + * If inc has bit set, decrement an additional bit if + * there is _no_ bit transition between orig and res. + * Else, inc has bit cleared, decrement an additional + * bit if there is a bit transition between orig and + * res. + */ + if ((inc ^ orig ^ res) & bit_mask) + inc -= bit_mask; + } else { + inc &= ~(bit_mask - 1); + /* + * xor bit_mask: overflow. + * + * If inc has bit set, increment an additional bit if + * there is _no_ bit transition between orig and res. + * Else, inc has bit cleared, increment an additional + * bit if there is a bit transition between orig and + * res. + */ + if ((inc ^ orig ^ res) & bit_mask) + inc += bit_mask; + } + return inc; +} + +static inline +void percpu_counter_tree_add(struct percpu_counter_tree *counter, int inc) +{ + unsigned int bit_mask = counter->level0_bit_mask, orig, res; + + if (!inc) + return; + /* Make sure the fast and slow paths use the same cpu number. */ + guard(migrate)(); + res = this_cpu_add_return(*counter->level0, inc); + orig = res - inc; + inc = percpu_counter_tree_carry(orig, res, inc, bit_mask); + if (!inc) + return; + percpu_counter_tree_add_slowpath(counter, inc); +} + +static inline +int percpu_counter_tree_approximate_sum(struct percpu_counter_tree *counter) +{ + unsigned int v; + + if (!counter->level0_bit_mask) + v = READ_ONCE(*counter->approx_sum.i); + else + v = atomic_read(counter->approx_sum.a); + return (int) (v + (unsigned int)READ_ONCE(counter->bias)); +} + +#endif /* _PERCPU_COUNTER_TREE_H */ diff --git a/lib/Makefile b/lib/Makefile index d5cfc7afbbb8..d803a3a63288 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -201,6 +201,7 @@ obj-$(CONFIG_TEXTSEARCH_KMP) += ts_kmp.o obj-$(CONFIG_TEXTSEARCH_BM) += ts_bm.o obj-$(CONFIG_TEXTSEARCH_FSM) += ts_fsm.o obj-$(CONFIG_SMP) += percpu_counter.o +obj-$(CONFIG_SMP) += percpu_counter_tree.o obj-$(CONFIG_AUDIT_GENERIC) += audit.o obj-$(CONFIG_AUDIT_COMPAT_GENERIC) += compat_audit.o diff --git a/lib/percpu_counter_tree.c b/lib/percpu_counter_tree.c new file mode 100644 index 000000000000..928df7ac601e --- /dev/null +++ b/lib/percpu_counter_tree.c @@ -0,0 +1,393 @@ +// SPDX-License-Identifier: GPL-2.0+ OR MIT +// SPDX-FileCopyrightText: 2025 Mathieu Desnoyers + +/* + * Split Counters With Tree Approximation Propagation + * + * * Propagation diagram when reaching batch size thresholds (± batch size): + * + * Example diagram for 8 CPUs: + * + * log2(8) = 3 levels + * + * At each level, each pair propagates its values to the next level when + * reaching the batch size thresholds. + * + * Counters at levels 0, 1, 2 can be kept on a single byte (±128 range), + * although it may be relevant to keep them on 32-bit counters for + * simplicity. (complexity vs memory footprint tradeoff) + * + * Counter at level 3 can be kept on a 32-bit counter. + * + * Level 0: 0 1 2 3 4 5 6 7 + * | / | / | / | / + * | / | / | / | / + * | / | / | / | / + * Level 1: 0 1 2 3 + * | / | / + * | / | / + * | / | / + * Level 2: 0 1 + * | / + * | / + * | / + * Level 3: 0 + * + * * Approximation inaccuracy: + * + * BATCH(level N): Level N batch size. + * + * Example for BATCH(level 0) = 32. + * + * BATCH(level 0) = 32 + * BATCH(level 1) = 64 + * BATCH(level 2) = 128 + * BATCH(level N) = BATCH(level 0) * 2^N + * + * per-counter global + * inaccuracy inaccuracy + * Level 0: [ -32 .. +31] ±256 (8 * 32) + * Level 1: [ -64 .. +63] ±256 (4 * 64) + * Level 2: [-128 .. +127] ±256 (2 * 128) + * Total: ------ ±768 (log2(nr_cpu_ids) * BATCH(level 0) * nr_cpu_ids) + * + * ----- + * + * Approximate Sum Carry Propagation + * + * Let's define a number of counter bits for each level, e.g.: + * + * log2(BATCH(level 0)) = log2(32) = 5 + * + * nr_bit value_mask range + * Level 0: 5 bits v 0 .. +31 + * Level 1: 1 bit (v & ~((1UL << 5) - 1)) 0 .. +63 + * Level 2: 1 bit (v & ~((1UL << 6) - 1)) 0 .. +127 + * Level 3: 25 bits (v & ~((1UL << 7) - 1)) 0 .. 2^32-1 + * + * Note: Use a full 32-bit per-cpu counter at level 0 to allow precise sum. + * + * Note: Use cacheline aligned counters at levels above 0 to prevent false sharing. + * If memory footprint is an issue, a specialized allocator could be used + * to eliminate padding. + * + * Example with expanded values: + * + * counter_add(counter, inc): + * + * if (!inc) + * return; + * + * res = percpu_add_return(counter @ Level 0, inc); + * orig = res - inc; + * if (inc < 0) { + * inc = -(-inc & ~0b00011111); // Clear used bits + * // xor bit 5: underflow + * if ((inc ^ orig ^ res) & 0b00100000) + * inc -= 0b00100000; + * } else { + * inc &= ~0b00011111; // Clear used bits + * // xor bit 5: overflow + * if ((inc ^ orig ^ res) & 0b00100000) + * inc += 0b00100000; + * } + * if (!inc) + * return; + * + * res = atomic_add_return(counter @ Level 1, inc); + * orig = res - inc; + * if (inc < 0) { + * inc = -(-inc & ~0b00111111); // Clear used bits + * // xor bit 6: underflow + * if ((inc ^ orig ^ res) & 0b01000000) + * inc -= 0b01000000; + * } else { + * inc &= ~0b00111111; // Clear used bits + * // xor bit 6: overflow + * if ((inc ^ orig ^ res) & 0b01000000) + * inc += 0b01000000; + * } + * if (!inc) + * return; + * + * res = atomic_add_return(counter @ Level 2, inc); + * orig = res - inc; + * if (inc < 0) { + * inc = -(-inc & ~0b01111111); // Clear used bits + * // xor bit 7: underflow + * if ((inc ^ orig ^ res) & 0b10000000) + * inc -= 0b10000000; + * } else { + * inc &= ~0b01111111; // Clear used bits + * // xor bit 7: overflow + * if ((inc ^ orig ^ res) & 0b10000000) + * inc += 0b10000000; + * } + * if (!inc) + * return; + * + * atomic_add(counter @ Level 3, inc); + */ + +#include +#include +#include +#include +#include +#include +#include +#include + +#define MAX_NR_LEVELS 5 + +struct counter_config { + unsigned int nr_items; + unsigned char nr_levels; + unsigned char n_arity_order[MAX_NR_LEVELS]; +}; + +/* + * nr_items is the number of items in the tree for levels 1 to and + * including the final level (approximate sum). It excludes the level 0 + * per-cpu counters. + */ +static const struct counter_config per_nr_cpu_order_config[] = { + [0] = { .nr_items = 1, .nr_levels = 0, .n_arity_order = { 0 } }, + [1] = { .nr_items = 3, .nr_levels = 1, .n_arity_order = { 1 } }, + [2] = { .nr_items = 3, .nr_levels = 2, .n_arity_order = { 1, 1 } }, + [3] = { .nr_items = 7, .nr_levels = 3, .n_arity_order = { 1, 1, 1 } }, + [4] = { .nr_items = 7, .nr_levels = 3, .n_arity_order = { 2, 1, 1 } }, + [5] = { .nr_items = 11, .nr_levels = 3, .n_arity_order = { 2, 2, 1 } }, + [6] = { .nr_items = 21, .nr_levels = 3, .n_arity_order = { 2, 2, 2 } }, + [7] = { .nr_items = 21, .nr_levels = 3, .n_arity_order = { 3, 2, 2 } }, + [8] = { .nr_items = 37, .nr_levels = 3, .n_arity_order = { 3, 3, 2 } }, + [9] = { .nr_items = 73, .nr_levels = 3, .n_arity_order = { 3, 3, 3 } }, + [10] = { .nr_items = 149, .nr_levels = 4, .n_arity_order = { 3, 3, 2, 2 } }, + [11] = { .nr_items = 293, .nr_levels = 4, .n_arity_order = { 3, 3, 3, 2 } }, + [12] = { .nr_items = 585, .nr_levels = 4, .n_arity_order = { 3, 3, 3, 3 } }, + [13] = { .nr_items = 1173, .nr_levels = 5, .n_arity_order = { 3, 3, 3, 2, 2 } }, + [14] = { .nr_items = 2341, .nr_levels = 5, .n_arity_order = { 3, 3, 3, 3, 2 } }, + [15] = { .nr_items = 4681, .nr_levels = 5, .n_arity_order = { 3, 3, 3, 3, 3 } }, + [16] = { .nr_items = 4681, .nr_levels = 5, .n_arity_order = { 4, 3, 3, 3, 3 } }, + [17] = { .nr_items = 8777, .nr_levels = 5, .n_arity_order = { 4, 4, 3, 3, 3 } }, + [18] = { .nr_items = 17481, .nr_levels = 5, .n_arity_order = { 4, 4, 4, 3, 3 } }, + [19] = { .nr_items = 34953, .nr_levels = 5, .n_arity_order = { 4, 4, 4, 4, 3 } }, + [20] = { .nr_items = 69905, .nr_levels = 5, .n_arity_order = { 4, 4, 4, 4, 4 } }, +}; + +static const struct counter_config *counter_config; +static unsigned int nr_cpus_order, inaccuracy_multiplier; + +int percpu_counter_tree_init(struct percpu_counter_tree *counter, unsigned int batch_size) +{ + /* Batch size must be power of 2 */ + if (!batch_size || (batch_size & (batch_size - 1))) + return -EINVAL; + counter->batch_size = batch_size; + counter->bias = 0; + counter->level0 = alloc_percpu(unsigned int); + if (!counter->level0) + return -ENOMEM; + if (!nr_cpus_order) { + counter->items = NULL; + counter->approx_sum.i = per_cpu_ptr(counter->level0, 0); + counter->level0_bit_mask = 0; + } else { + counter->items = kzalloc(counter_config->nr_items * + sizeof(struct percpu_counter_tree_level_item), + GFP_KERNEL); + if (!counter->items) { + free_percpu(counter->level0); + return -ENOMEM; + } + counter->approx_sum.a = &counter->items[counter_config->nr_items - 1].count; + counter->level0_bit_mask = 1UL << get_count_order(batch_size); + } + counter->inaccuracy = batch_size * inaccuracy_multiplier; + return 0; +} + +void percpu_counter_tree_destroy(struct percpu_counter_tree *counter) +{ + free_percpu(counter->level0); + kfree(counter->items); +} + +/* Called with migration disabled. */ +void percpu_counter_tree_add_slowpath(struct percpu_counter_tree *counter, int inc) +{ + unsigned int level_items, nr_levels = counter_config->nr_levels, + level, n_arity_order, bit_mask; + struct percpu_counter_tree_level_item *item = counter->items; + unsigned int cpu = smp_processor_id(); + + WARN_ON_ONCE(!nr_cpus_order); /* Should never be called for 1 cpu. */ + + n_arity_order = counter_config->n_arity_order[0]; + bit_mask = counter->level0_bit_mask << n_arity_order; + level_items = 1U << (nr_cpus_order - n_arity_order); + + for (level = 1; level < nr_levels; level++) { + atomic_t *count = &item[cpu & (level_items - 1)].count; + unsigned int orig, res; + + res = atomic_add_return_relaxed(inc, count); + orig = res - inc; + inc = percpu_counter_tree_carry(orig, res, inc, bit_mask); + if (!inc) + return; + item += level_items; + n_arity_order = counter_config->n_arity_order[level]; + level_items >>= n_arity_order; + bit_mask <<= n_arity_order; + } + atomic_add(inc, counter->approx_sum.a); +} + +/* + * Precise sum. Perform the sum of all per-cpu counters. + */ +static int percpu_counter_tree_precise_sum_unbiased(struct percpu_counter_tree *counter) +{ + unsigned int sum = 0; + int cpu; + + for_each_possible_cpu(cpu) + sum += *per_cpu_ptr(counter->level0, cpu); + return (int) sum; +} + +int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter) +{ + return percpu_counter_tree_precise_sum_unbiased(counter) + READ_ONCE(counter->bias); +} + +/* + * Do an approximate comparison of two counters. + * Return 0 if counters do not differ by more than the sum of their + * respective inaccuracy ranges, + * Return -1 if counter @a less than counter @b, + * Return 1 if counter @a is greater than counter @b. + */ +int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b) +{ + int count_a = percpu_counter_tree_approximate_sum(a), + count_b = percpu_counter_tree_approximate_sum(b); + + if (abs(count_a - count_b) <= (a->inaccuracy + b->inaccuracy)) + return 0; + if (count_a < count_b) + return -1; + return 1; +} + +/* + * Do an approximate comparison of a counter against a given value. + * Return 0 if the value is within the inaccuracy range of the counter, + * Return -1 if the value less than counter, + * Return 1 if the value is greater than counter. + */ +int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v) +{ + int count = percpu_counter_tree_approximate_sum(counter); + + if (abs(v - count) <= counter->inaccuracy) + return 0; + if (count < v) + return -1; + return 1; +} + +/* + * Do a precise comparison of two counters. + * Return 0 if the counters are equal, + * Return -1 if counter @a less than counter @b, + * Return 1 if counter @a is greater than counter @b. + */ +int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b) +{ + int count_a = percpu_counter_tree_approximate_sum(a), + count_b = percpu_counter_tree_approximate_sum(b); + + if (abs(count_a - count_b) <= (a->inaccuracy + b->inaccuracy)) { + if (b->inaccuracy < a->inaccuracy) { + count_a = percpu_counter_tree_precise_sum(a); + if (abs(count_a - count_b) <= b->inaccuracy) + count_b = percpu_counter_tree_precise_sum(b); + } else { + count_b = percpu_counter_tree_precise_sum(b); + if (abs(count_a - count_b) <= a->inaccuracy) + count_a = percpu_counter_tree_precise_sum(a); + } + } + if (count_a > count_b) + return -1; + if (count_a > count_b) + return 1; + return 0; +} + +/* + * Do a precise comparision of a counter against a given value. + * Return 0 if the value is equal to the counter, + * Return -1 if the value less than counter, + * Return 1 if the value is greater than counter. + */ +int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v) +{ + int count = percpu_counter_tree_approximate_sum(counter); + + if (abs(v - count) <= counter->inaccuracy) + count = percpu_counter_tree_precise_sum(counter); + if (count < v) + return -1; + if (count > v) + return 1; + return 0; +} + +void percpu_counter_tree_set_bias(struct percpu_counter_tree *counter, int bias) +{ + WRITE_ONCE(counter->bias, bias); +} + +void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v) +{ + percpu_counter_tree_set_bias(counter, + v - percpu_counter_tree_precise_sum_unbiased(counter)); +} + +unsigned int percpu_counter_tree_inaccuracy(struct percpu_counter_tree *counter) +{ + return counter->inaccuracy; +} + +static unsigned int __init calculate_inaccuracy_multiplier(void) +{ + unsigned int nr_levels = counter_config->nr_levels, level; + unsigned int level_items = 1U << nr_cpus_order; + unsigned int inaccuracy = 0, batch_size = 1; + + for (level = 0; level < nr_levels; level++) { + unsigned int n_arity_order = counter_config->n_arity_order[level]; + + inaccuracy += batch_size * level_items; + batch_size <<= n_arity_order; + level_items >>= n_arity_order; + } + return inaccuracy; +} + +static int __init percpu_counter_startup(void) +{ + + nr_cpus_order = get_count_order(nr_cpu_ids); + if (WARN_ON_ONCE(nr_cpus_order >= ARRAY_SIZE(per_nr_cpu_order_config))) { + printk(KERN_ERR "Unsupported number of CPUs (%u)\n", nr_cpu_ids); + return -1; + } + counter_config = &per_nr_cpu_order_config[nr_cpus_order]; + inaccuracy_multiplier = calculate_inaccuracy_multiplier(); + return 0; +} +module_init(percpu_counter_startup); -- 2.39.5