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From: Hao Li To: Zhao Liu Cc: Vlastimil Babka , Hao Li , akpm@linux-foundation.org, harry.yoo@oracle.com, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, tim.c.chen@intel.com, yu.c.chen@intel.com Subject: Re: [PATCH v2] slub: keep empty main sheaf as spare in __pcs_replace_empty_main() Message-ID: <3foece42fc3wj3wgcnibi7cn2be73ancyjgrzdqfjeqvjtwkhj@lzqd572wfpvz> References: <20251210002629.34448-1-haoli.tcs@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: E44CC1C0004 X-Stat-Signature: ze685pze85uhrykjar8ptxn15o9xzjhf X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1768536385-640040 X-HE-Meta: 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 NTXOLO/7 6e7iMWIa67buFFcWmtWJ+SYK59WBP/PW6QdxihTzQuSjAS0wH55V32U0EdT6yyqt4+KmKCr9qywb80gb4w4u0u7tF752zOZSJT0IBVfGvuVvD3UyPGv35ni2XlMy6pN4S0snWSH/SyWm/7I2FzAWpQ/DjBPo/p1ajuf6Lnt85N0Qinul8RCOzNEvaCXBXx9A42hZ6LzE1wYN5k9jA9ol1esiMRHIaSP3sNA0QtY3J4b0NtKhYVkZ87rrmOZOqVOGfUHnrQSJu+fDiqsg1Wa0CaKcq+EXjTr4eNk/kiYVR7lGxSSE9Lual7t1xuXMm/6xNpi9anpRq6aqG9YrCyk46xTJ9rLySjrO0/yE0 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: On Thu, Jan 15, 2026 at 06:12:44PM +0800, Zhao Liu wrote: > Hi Babka & Hao, > > > Thanks, LGTM. We can make it smaller though. Adding to slab/for-next > > adjusted like this: > > > > diff --git a/mm/slub.c b/mm/slub.c > > index f21b2f0c6f5a..ad71f01571f0 100644 > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -5052,7 +5052,11 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs, > > */ > > > > if (pcs->main->size == 0) { > > - barn_put_empty_sheaf(barn, pcs->main); > > + if (!pcs->spare) { > > + pcs->spare = pcs->main; > > + } else { > > + barn_put_empty_sheaf(barn, pcs->main); > > + } > > pcs->main = full; > > return pcs; > > } > > I noticed the previous lkp regression report and tested this fix: > > * will-it-scale.per_process_ops > > Compared with v6.19-rc4(f0b9d8eb98df), with this fix, I have these > results: > > nr_tasks Delta > 1 + 3.593% > 8 + 3.094% > 64 +60.247% > 128 +49.344% > 192 +27.500% > 256 -12.077% > > For the cases (nr_tasks: 1-192), there're the improvements. I think > this is expected since pre-cached spare sheaf reduces spinlock race: > reduce barn_put_empty_sheaf() & barn_get_empty_sheaf(). > > So (maybe too late), > > Tested-by: Zhao Liu Hello, Zhao, Thanks for running the performance test! > > > > But I find there are two more questions that might need consideration? > > # Question 1: Regression for 256 tasks > > For the above test - the case with nr_tasks: 256, there's a "slight" > regression. I did more testing: > > (This is a single-round test; the 256-tasks data has jitter.) > > nr_tasks Delta > 244 0.308% > 248 - 0.805% > 252 12.070% > 256 -11.441% > 258 2.070% > 260 1.252% > 264 2.369% > 268 -11.479% > 272 2.130% > 292 8.714% > 296 10.905% > 298 17.196% > 300 11.783% > 302 6.620% > 304 3.112% > 308 - 5.924% > > It can be seen that most cases show improvement, though a few may > experience slight regression. > > Based on the configuration of my machine: > > GNR - 2 sockets with the following NUMA topology: > > NUMA: > NUMA node(s): 4 > NUMA node0 CPU(s): 0-42,172-214 > NUMA node1 CPU(s): 43-85,215-257 > NUMA node2 CPU(s): 86-128,258-300 > NUMA node3 CPU(s): 129-171,301-343 > > Since I set the CPU affinity on the core, 256 cases is roughly > equivalent to the moment when Node 0 and Node 1 are filled. > > The following is the perf data comparing 2 tests w/o fix & with this fix: > > # Baseline Delta Abs Shared Object Symbol > # ........ ......... ....................... .................................... > # > 61.76% +4.78% [kernel.vmlinux] [k] native_queued_spin_lock_slowpath > 0.93% -0.32% [kernel.vmlinux] [k] __slab_free > 0.39% -0.31% [kernel.vmlinux] [k] barn_get_empty_sheaf > 1.35% -0.30% [kernel.vmlinux] [k] mas_leaf_max_gap > 3.22% -0.30% [kernel.vmlinux] [k] __kmem_cache_alloc_bulk > 1.73% -0.20% [kernel.vmlinux] [k] __cond_resched > 0.52% -0.19% [kernel.vmlinux] [k] _raw_spin_lock_irqsave > 0.92% +0.18% [kernel.vmlinux] [k] _raw_spin_lock > 1.91% -0.15% [kernel.vmlinux] [k] zap_pmd_range.isra.0 > 1.37% -0.13% [kernel.vmlinux] [k] mas_wr_node_store > 1.29% -0.12% [kernel.vmlinux] [k] free_pud_range > 0.92% -0.11% [kernel.vmlinux] [k] __mmap_region > 0.12% -0.11% [kernel.vmlinux] [k] barn_put_empty_sheaf > 0.20% -0.09% [kernel.vmlinux] [k] barn_replace_empty_sheaf > 0.31% +0.09% [kernel.vmlinux] [k] get_partial_node > 0.29% -0.07% [kernel.vmlinux] [k] __rcu_free_sheaf_prepare > 0.12% -0.07% [kernel.vmlinux] [k] intel_idle_xstate > 0.21% -0.07% [kernel.vmlinux] [k] __kfree_rcu_sheaf > 0.26% -0.07% [kernel.vmlinux] [k] down_write > 0.53% -0.06% libc.so.6 [.] __mmap > 0.66% -0.06% [kernel.vmlinux] [k] mas_walk > 0.48% -0.06% [kernel.vmlinux] [k] mas_prev_slot > 0.45% -0.06% [kernel.vmlinux] [k] mas_find > 0.38% -0.06% [kernel.vmlinux] [k] mas_wr_store_type > 0.23% -0.06% [kernel.vmlinux] [k] do_vmi_align_munmap > 0.21% -0.05% [kernel.vmlinux] [k] perf_event_mmap_event > 0.32% -0.05% [kernel.vmlinux] [k] entry_SYSRETQ_unsafe_stack > 0.19% -0.05% [kernel.vmlinux] [k] downgrade_write > 0.59% -0.05% [kernel.vmlinux] [k] mas_next_slot > 0.31% -0.05% [kernel.vmlinux] [k] __mmap_new_vma > 0.44% -0.05% [kernel.vmlinux] [k] kmem_cache_alloc_noprof > 0.28% -0.05% [kernel.vmlinux] [k] __vma_enter_locked > 0.41% -0.05% [kernel.vmlinux] [k] memcpy > 0.48% -0.04% [kernel.vmlinux] [k] mas_store_gfp > 0.14% +0.04% [kernel.vmlinux] [k] __put_partials > 0.19% -0.04% [kernel.vmlinux] [k] mas_empty_area_rev > 0.30% -0.04% [kernel.vmlinux] [k] do_syscall_64 > 0.25% -0.04% [kernel.vmlinux] [k] mas_preallocate > 0.15% -0.04% [kernel.vmlinux] [k] rcu_free_sheaf > 0.22% -0.04% [kernel.vmlinux] [k] entry_SYSCALL_64 > 0.49% -0.04% libc.so.6 [.] __munmap > 0.91% -0.04% [kernel.vmlinux] [k] rcu_all_qs > 0.21% -0.04% [kernel.vmlinux] [k] __vm_munmap > 0.24% -0.04% [kernel.vmlinux] [k] mas_store_prealloc > 0.19% -0.04% [kernel.vmlinux] [k] __kmalloc_cache_noprof > 0.34% -0.04% [kernel.vmlinux] [k] build_detached_freelist > 0.19% -0.03% [kernel.vmlinux] [k] vms_complete_munmap_vmas > 0.36% -0.03% [kernel.vmlinux] [k] mas_rev_awalk > 0.05% -0.03% [kernel.vmlinux] [k] shuffle_freelist > 0.19% -0.03% [kernel.vmlinux] [k] down_write_killable > 0.19% -0.03% [kernel.vmlinux] [k] kmem_cache_free > 0.27% -0.03% [kernel.vmlinux] [k] up_write > 0.13% -0.03% [kernel.vmlinux] [k] vm_area_alloc > 0.18% -0.03% [kernel.vmlinux] [k] arch_get_unmapped_area_topdown > 0.08% -0.03% [kernel.vmlinux] [k] userfaultfd_unmap_complete > 0.10% -0.03% [kernel.vmlinux] [k] tlb_gather_mmu > 0.30% -0.02% [kernel.vmlinux] [k] ___slab_alloc > > I think the insteresting item is "get_partial_node". It seems this fix > makes "get_partial_node" slightly more frequent. HMM, however, I still > can't figure out why this is happening. Do you have any thoughts on it? I'd like to dig a bit deeper to confirm whether the "256 tasks" result is truly a regression. Could you please share the original full report, or let me know which test case under will-it-scale/ you used? -- Thanks, Hao > > # Question 2: sheaf capacity > > Back the original commit which triggerred lkp regression. I did more > testing to check if this fix could totally fill the regression gap. > > The base line is commit 3accabda4 ("mm, vma: use percpu sheaves for > vm_area_struct cache") and its next commit 59faa4da7cd4 ("maple_tree: > use percpu sheaves for maple_node_cache") has the regression. > > I compared v6.19-rc4(f0b9d8eb98df) w/o fix & with fix aginst the base > line: > > nr_tasks w/o fix with fix > 1 - 3.643% - 0.181% > 8 -12.523% - 9.816% > 64 -50.378% -20.482% > 128 -36.736% - 5.518% > 192 -22.963% - 1.777% > 256 -32.926% - 41.026% > > It appears that under extreme conditions, regression remains significate. > I remembered your suggestion about larger capacity and did the following > testing: > > 59faa4da7cd4 59faa4da7cd4 59faa4da7cd4 59faa4da7cd4 59faa4da7cd4 > (with this fix) (cap: 32->64) (cap: 32->128) (cap: 32->256) > 1 -8.789% -8.805% -8.185% -9.912% -8.673% > 8 -12.256% -9.219% -10.460% -10.070% -8.819% > 64 -38.915% -8.172% -4.700% 4.571% 8.793% > 128 -8.032% 11.377% 23.232% 26.940% 30.573% > 192 -1.220% 9.758% 20.573% 22.645% 25.768% > 256 -6.570% 9.967% 21.663% 30.103% 33.876% > > Comparing with base line (3accabda4), larger capacity could > significatly improve the Sheaf's scalability. > > So, I'd like to know if you think dynamically or adaptively adjusting > capacity is a worthwhile idea. > > Thanks for your patience. > > Regards, > Zhao > >