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wrote: > On Tue, Feb 24, 2026 at 10:52:28AM +0800, Ming Lei wrote: > > Reproducer > > ========== > > > [...] > > > > the result is that the allocating cpu's per-cpu slab caches are > > continuously drained without being replenished by local frees. the bio > > layer's own per-cpu cache (bio_alloc_cache) suffers the same mismatch: > > freed bios go to the completion cpu's cache via bio_put_percpu_cache(), > > leaving the submitter cpus' caches empty and falling through to > > mempool_alloc() -> kmem_cache_alloc() -> slub slow path. > > > > in v6.19, slub handled this with a 3-tier allocation hierarchy: > > > > Tier 1: CPU slab freelist lock-free (cmpxchg) > > Tier 2: CPU partial slab list lock-free (per-CPU local_lock) > > Tier 3: Node partial list kmem_cache_node->list_lock > > > > The CPU partial slab list (Tier 2) was the critical buffer. It was > > populated during __slab_free() -> put_cpu_partial() and provided a > > lock-free pool of partial slabs per CPU. Even when the CPU slab was > > exhausted, the CPU partial list could supply more slabs without > > touching any shared lock. > > > > The sheaves architecture replaces this with a 2-tier hierarchy: > > > > Tier 1: Per-CPU sheaf lock-free (local_lock) > > Tier 2: Node partial list kmem_cache_node->list_lock > > > > The intermediate lock-free tier is gone. When the per-CPU sheaf is > > empty and the spare sheaf is also empty, every refill must go through > > the node partial list, requiring kmem_cache_node->list_lock. With 16 > > CPUs simultaneously allocating bios and all hitting empty sheaves, this > > creates a thundering herd on the node list_lock. > > > > When the local node's partial list is also depleted (objects freed on > > remote nodes accumulate there instead), get_from_any_partial() kicks in > > to search other NUMA nodes, compounding the contention with cross-NUMA > > list_lock acquisition — explaining the 41% in get_from_any_partial -> > > native_queued_spin_lock_slowpath seen in the profile. > > The purpose of introducing sheaves was to fully replace the percpu partial slabs > mechanism with sheaves. During this process, we first added the sheaves caching > layer and only later removed the percpu partial slabs layer, so it's expected > that performance could first improve and then return to the previous level. There's one difference here; you used will-it-scale mmap2 test case that involves maple tree node and vm_area_struct cache that already has sheaves enabled in v6.19. And Ming's benchmark stresses bio- caches. Since other caches don't have sheaves in v6.19, they're not supposed to have performance gain by having additional sheaves layer on top of cpu slab + percpu partial slab list. > Would you mind also comparing against a baseline with "no sheaves at all" (e.g. > commit `9d4e6ab865c4`) versus "only the sheaves layer exists" (i.e. commit > `815c8e35511d`)? If those two results are close, then the ~64% performance > regression we're currently discussing might be better interpreted as returning > to the previous baseline (i.e. a reversion), rather than a true regression. > > The link below contains my previous test results. According to will-it-scale, > the performance of "no sheaves at all" and "only the sheaves layer exists" is > close: > https://lore.kernel.org/linux-mm/pdmjsvpkl5nsntiwfwguplajq27ak3xpboq3ab77zrbu763pq7@la3hyiqigpir/ -- Cheers, Harry / Hyeonggon