> On Oct 9, 2026, at 11:07 PM, KunWu Chan <[email protected]> wrote: > > On Tue, Oct 6, 2026 at 1:51 PM Mathieu Desnoyers > <[email protected]> wrote: >> >>> On 2026-10-05 13:15, Kunwu Chan wrote: >>> Replace the per-waiter exact-match scan with a shared Bloom filter >>> built during the second drain pass: each observed non-wildcard >>> address sets three bits, so a waiter whose address hashes to an >>> unset bit can be completed without another slot walk. >>> >>> A Bloom filter has no false negatives, so an address absent from >>> the final drain scan cannot be held by an observed slot; false >>> positives only delay completion to a later scan cycle. >>> >>> The filter uses a one-page bitmap with three multiply-shift hash >>> functions. It lives in the hazptr_scan_state and is only touched >>> by the scan kthread, so waiter state remains on the caller's stack >>> and no dynamic allocation is needed. >>> >>> Suggested-by: Boqun Feng <[email protected]> >> >> I actually suggested that to Boqun in the first place. Feel >> free to add my suggested-by tag as well. :) >> >> Did you run your Bloom filter in a test program on the side to >> see how it behaves with pointers, and confirm that the false >> positive rate is not too high due to hashing algorithm/constants >> concerns ? >> > > Hi Mathieu, > > I ran a standalone test using the hash functions and > constants from > the patch, with random, > page-aligned, cacheline-aligned, sequential, and two synthetic > kernel-like address patterns.
kernel-like? Linux or CUDA? I have to say I find AI generated messages hard to read. > > With 768 inserted addresses, the measured false-positive rates were > 0.025%–0.034% across > these patterns, compared with an ideal Bloom-filter estimate of about > 0.031%. I also tested larger > insertion counts as stress cases. The results were consistent across > three seeds, > with only minor variation for the random distribution. Why is the distribution random? > > These are synthetic address distributions rather than measurements > from actual kernel waiter > addresses, but they provide an initial check of the hash functions > against several structured > address patterns. Why not just provide the test code? Oh, but your AI crutches did not ask you to do that. Joel > > Thanks, > Kunwu > >> Thanks! >> >> Mathieu >> >>> Signed-off-by: Kunwu Chan <[email protected]> >>> --- >>> kernel/hazptr.c | 93 +++++++++++++++++++++++++++++++++++++++++++------ >>> 1 file changed, 83 insertions(+), 10 deletions(-) >>> >>> diff --git a/kernel/hazptr.c b/kernel/hazptr.c >>> index 9e274a691af5..900ba35de2cb 100644 >>> --- a/kernel/hazptr.c >>> +++ b/kernel/hazptr.c >>> @@ -7,6 +7,7 @@ >>> */ >>> >>> #include <linux/hazptr.h> >>> +#include <linux/bitops.h> >>> #include <linux/percpu.h> >>> #include <linux/spinlock.h> >>> #include <linux/mutex.h> >>> @@ -16,6 +17,7 @@ >>> #include <linux/kthread.h> >>> #include <linux/swait.h> >>> #include <linux/sched.h> >>> +#include <linux/bitmap.h> >>> >>> /* >>> * The current hazard pointer wildcard. Flips between 1UL and 2UL to >>> guarantee >>> @@ -213,6 +215,55 @@ void hazptr_scan_period(void *addr, void >>> *scan_wildcard) >>> } >>> } >>> >>> +/* Number of hash functions for the Bloom filter. */ >>> +#define HAZPTR_BLOOM_HASHES 3 >>> + >>> +/* Number of bits in the Bloom filter bitmap: one page. */ >>> +#define HAZPTR_BLOOM_NBITS (PAGE_SIZE * 8) >>> + >>> +struct hazptr_bloom { >>> + unsigned long map[PAGE_SIZE / sizeof(unsigned long)]; >>> +}; >>> + >>> +/* >>> + * Multiply @addr by a per-hash odd constant and use the high bits >>> + * as the Bloom filter index. >>> + */ >>> +static unsigned long hazptr_bloom_hash(void *addr, unsigned int i) >>> +{ >>> + static const u64 mult[HAZPTR_BLOOM_HASHES] = { >>> + 0x9E3779B97F4A7C15ULL, >>> + 0xC2B2AE3D27D4EB4FULL, >>> + 0x165667B19E3779F9ULL, >>> + }; >>> + u64 hash = (u64)(unsigned long)addr * mult[i]; >>> + >>> + return hash >> (64 - ilog2(HAZPTR_BLOOM_NBITS)); >>> +} >>> + >>> +static void hazptr_bloom_reset(struct hazptr_bloom *bloom) >>> +{ >>> + bitmap_zero(bloom->map, HAZPTR_BLOOM_NBITS); >>> +} >>> + >>> +static void hazptr_bloom_add(struct hazptr_bloom *bloom, void *addr) >>> +{ >>> + unsigned int i; >>> + >>> + for (i = 0; i < HAZPTR_BLOOM_HASHES; i++) >>> + __set_bit(hazptr_bloom_hash(addr, i), bloom->map); >>> +} >>> + >>> +static bool hazptr_bloom_contains(const struct hazptr_bloom *bloom, void >>> *addr) >>> +{ >>> + unsigned int i; >>> + >>> + for (i = 0; i < HAZPTR_BLOOM_HASHES; i++) >>> + if (!test_bit(hazptr_bloom_hash(addr, i), bloom->map)) >>> + return false; >>> + return true; >>> +} >>> + >>> struct hazptr_waiter { >>> struct list_head node; >>> void *addr; >>> @@ -226,17 +277,26 @@ struct hazptr_scan_state { >>> struct mutex lock; >>> struct list_head pending; >>> struct list_head scanning; /* kthread only */ >>> + struct hazptr_bloom bloom; /* kthread only */ >>> }; >>> static struct hazptr_scan_state hazptr_scan; >>> >>> /* >>> - * Check per-CPU slots before overflow-list slots to match the >>> - * acquisition ordering of promoted slots. >>> + * Walk all slots and return true if @watch is present. If @bloom >>> + * is non-NULL, record observed non-wildcard addresses in it. >>> + * >>> + * Per-CPU slots are examined before overflow-list slots on each CPU >>> + * to preserve the acquisition ordering required by the promote path: >>> + * synchronize must observe the per-CPU slot release before the >>> + * overflow-list entry can be missed. >>> */ >>> -static bool hazptr_value_present(void *val) >>> +static bool hazptr_scan_walk(void *watch, struct hazptr_bloom *bloom) >>> { >>> int cpu; >>> >>> + if (bloom) >>> + hazptr_bloom_reset(bloom); >>> + >>> for_each_possible_cpu(cpu) { >>> struct hazptr_percpu_slots *percpu_slots = >>> per_cpu_ptr(&hazptr_percpu_slots, cpu); >>> struct hazptr_overflow_list_flip *overflow_list_flip = >>> per_cpu_ptr(&percpu_overflow_list_flip, cpu); >>> @@ -244,10 +304,14 @@ static bool hazptr_value_present(void *val) >>> >>> for (idx = 0; idx < NR_HAZPTR_PERCPU_SLOTS; idx++) { >>> struct hazptr_slot_item *item = >>> &percpu_slots->items[idx]; >>> + void *v; >>> >>> /* Pairs with smp_store_release in hazptr_release(). */ >>> - if (smp_load_acquire(&item->slot.addr) == val) >>> + v = smp_load_acquire(&item->slot.addr); >>> + if (v == watch) >>> return true; >>> + if (bloom && v && !is_wildcard(v)) >>> + hazptr_bloom_add(bloom, v); >>> } >>> for (int i = 0; i < 2; i++) { >>> struct hazptr_overflow_list *list = >>> &overflow_list_flip->array[i]; >>> @@ -256,11 +320,16 @@ static bool hazptr_value_present(void *val) >>> >>> raw_spin_lock_irqsave(&list->lock, flags); >>> hlist_for_each_entry(b, &list->head, overflow_node) { >>> + void *v; >>> + >>> /* Pairs with smp_store_release in >>> hazptr_release(). */ >>> - if (smp_load_acquire(&b->slot.addr) == val) { >>> + v = smp_load_acquire(&b->slot.addr); >>> + if (v == watch) { >>> >>> raw_spin_unlock_irqrestore(&list->lock, flags); >>> return true; >>> } >>> + if (bloom && v && !is_wildcard(v)) >>> + hazptr_bloom_add(bloom, v); >>> } >>> raw_spin_unlock_irqrestore(&list->lock, flags); >>> } >>> @@ -276,7 +345,7 @@ static bool hazptr_value_present(void *val) >>> */ >>> static void hazptr_drain_wildcard(void *wc) >>> { >>> - while (hazptr_value_present(wc)) >>> + while (hazptr_scan_walk(wc, NULL)) >>> cond_resched(); >>> } >>> >>> @@ -309,12 +378,16 @@ static void hazptr_scan_do_cycle(void) >>> WRITE_ONCE(hazptr_wildcard, scan_wildcard); >>> old_wildcard = flip_wildcard(scan_wildcard); >>> >>> - /* Pass 2: drain the old wildcard. */ >>> - hazptr_drain_wildcard(old_wildcard); >>> + /* >>> + * Pass 2: drain the old wildcard while collecting observed >>> + * addresses into the Bloom filter. >>> + */ >>> + while (hazptr_scan_walk(old_wildcard, &hazptr_scan.bloom)) >>> + cond_resched(); >>> >>> - /* Complete waiters whose address is no longer held by any slot. */ >>> + /* Complete waiters whose address is not in the filter. */ >>> list_for_each_entry_safe(w, n, &hazptr_scan.scanning, node) { >>> - if (!hazptr_value_present(w->addr)) >>> + if (!hazptr_bloom_contains(&hazptr_scan.bloom, w->addr)) >>> list_move(&w->node, &done); >>> } >>> >> >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. >> https://www.efficios.com
