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 m+I4e6Wf 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Feb 24, 2026 at 09:27:40PM +0100, Vlastimil Babka wrote: > On 2/24/26 3:52 AM, Ming Lei wrote: > > Hello Vlastimil and MM guys, > > > > The SLUB "sheaves" series merged via 815c8e35511d ("Merge branch > > 'slab/for-7.0/sheaves' into slab/for-next") introduces a severe > > performance regression for workloads with persistent cross-CPU > > alloc/free patterns. ublk null target benchmark IOPS drops > > significantly compared to v6.19: from ~36M IOPS to ~13M IOPS (~64% > > drop). > > > > Bisecting within the sheaves series is blocked by a kernel panic at > > 17c38c88294d ("slab: remove cpu (partial) slabs usage from allocation > > paths"), so the exact first bad commit could not be identified. > > > > Reproducer > > ========== > > > > Hardware: NUMA machine with >= 32 CPUs > > Kernel: v7.0-rc (with slab/for-7.0/sheaves merged) > > > > # build kublk selftest > > make -C tools/testing/selftests/ublk/ > > > > # create ublk null target device with 16 queues > > tools/testing/selftests/ublk/kublk add -t null -q 16 > > > > # run fio/t/io_uring benchmark: 16 jobs, 20 seconds, non-polled > > taskset -c 0-31 fio/t/io_uring -p0 -n 16 -r 20 /dev/ublkb0 > > > > # cleanup > > tools/testing/selftests/ublk/kublk del -n 0 > > > > Good: v6.19 (and 41f1a08645ab, the mainline parent of the slab merge) > > Bad: 815c8e35511d (Merge branch 'slab/for-7.0/sheaves' into slab/for-next) > > > > perf profile (bad kernel) > > ========================= > > > > ~47% of CPU time is spent in bio allocation hitting the SLUB slow path, > > with massive spinlock contention on the node partial list lock: > > > > + 47.65% 1.21% io_uring [k] bio_alloc_bioset > > - 44.87% 0.45% io_uring [k] kmem_cache_alloc_noprof > > - 44.41% kmem_cache_alloc_noprof > > - 43.89% ___slab_alloc > > + 41.16% get_from_any_partial > > So this function is not used in the sheaf refill path, but in the > fallback slowpath when alloc_from_pcs() fastpath fails. Good point. > > 0.91% get_from_partial_node > > + 0.87% alloc_from_new_slab > > + 0.65% allocate_slab > > - 44.70% 0.21% io_uring [k] mempool_alloc_noprof > > - 44.49% mempool_alloc_noprof > > - 44.43% kmem_cache_alloc_noprof > > And I'd guess alloc_from_pcs() fails because in > __pcs_replace_empty_main() we have gfpflags_allow_blocking() false, > because mempool_alloc_noprof() tries the first attempt without > __GFP_DIRECT_RECLAIM. So that will succeed, but we end up relying on the > slowpath all the time and performance will drop. That's very good point. I was missing that aspect. > It made sense to me not to refill sheaves when we can't reclaim, but I > didn't anticipate this interaction with mempools. Me neither :) > We could change them but there might be others using a similar pattern. Probably, yes. > Maybe it would be for the best to just drop that heuristic from > __pcs_replace_empty_main() Sounds fair. > (but carefully as some deadlock avoidance depends on it, we might need > to e.g. replace it with gfpflags_allow_spinning()). I'll send a patch > tomorrow to test this theory, unless someone beats me to it (feel free to). I think your point is valid. Let's give it a try. > Until then IMHO we can dismiss the AI explanation and also the > insufficient sheaf capacity theories. Yeah :) let's first see how it performs after addressing your point. -- Cheers, Harry / Hyeonggon