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Mon, 20 Oct 2025 11:23:31 -0700 (PDT) Date: Mon, 20 Oct 2025 11:23:29 -0700 From: "Vishal Moola (Oracle)" To: Uladzislau Rezki Cc: Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton Subject: Re: [RFC PATCH] mm/vmalloc: request large order pages from buddy allocator Message-ID: References: <20251014182754.4329-1-vishal.moola@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Oct 17, 2025 at 07:19:16PM +0200, Uladzislau Rezki wrote: > On Fri, Oct 17, 2025 at 06:15:21PM +0200, Uladzislau Rezki wrote: > > On Thu, Oct 16, 2025 at 12:02:59PM -0700, Vishal Moola (Oracle) wrote: > > > On Thu, Oct 16, 2025 at 10:42:04AM -0700, Vishal Moola (Oracle) wrote: > > > > On Thu, Oct 16, 2025 at 06:12:36PM +0200, Uladzislau Rezki wrote: > > > > > On Wed, Oct 15, 2025 at 02:28:49AM -0700, Vishal Moola (Oracle) wrote: > > > > > > On Wed, Oct 15, 2025 at 04:56:42AM +0100, Matthew Wilcox wrote: > > > > > > > On Tue, Oct 14, 2025 at 11:27:54AM -0700, Vishal Moola (Oracle) wrote: > > > > > > > > Running 1000 iterations of allocations on a small 4GB system finds: > > > > > > > > > > > > > > > > 1000 2mb allocations: > > > > > > > > [Baseline] [This patch] > > > > > > > > real 46.310s real 34.380s > > > > > > > > user 0.001s user 0.008s > > > > > > > > sys 46.058s sys 34.152s > > > > > > > > > > > > > > > > 10000 200kb allocations: > > > > > > > > [Baseline] [This patch] > > > > > > > > real 56.104s real 43.946s > > > > > > > > user 0.001s user 0.003s > > > > > > > > sys 55.375s sys 43.259s > > > > > > > > > > > > > > > > 10000 20kb allocations: > > > > > > > > [Baseline] [This patch] > > > > > > > > real 0m8.438s real 0m9.160s > > > > > > > > user 0m0.001s user 0m0.002s > > > > > > > > sys 0m7.936s sys 0m8.671s > > > > > > > > > > > > > > I'd be more confident in the 20kB numbers if you'd done 10x more > > > > > > > iterations. > > > > > > > > > > > > I actually ran my a number of times to mitigate the effects of possibly > > > > > > too small sample sizes, so I do have that number for you too: > > > > > > > > > > > > [Baseline] [This patch] > > > > > > real 1m28.119s real 1m32.630s > > > > > > user 0m0.012s user 0m0.011s > > > > > > sys 1m23.270s sys 1m28.529s > > > > > > > > > > > I have just had a look at performance figures of this patch. The test > > > > > case is 16K allocation by one single thread, 1 000 000 loops, 10 run: > > > > > > > > > > sudo ./test_vmalloc.sh run_test_mask=1 nr_threads=1 nr_pages=4 > > > > > > > > The reason I didn't use this test module is the same concern Matthew > > > > brought up earlier about testing the PCP list rather than buddy > > > > allocator. The test module allocates, then frees over and over again, > > > > making it incredibly prone to reuse the pages over and over again. > > > > > > > > > BOX: AMD Milan, 256 CPUs, 512GB of memory > > > > > > > > > > # default 16K alloc > > > > > [ 15.823704] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 955334 usec > > > > > [ 17.751685] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1158739 usec > > > > > [ 19.443759] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1016522 usec > > > > > [ 21.035701] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 911381 usec > > > > > [ 22.727688] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 987286 usec > > > > > [ 24.199694] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 955112 usec > > > > > [ 25.755675] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 926393 usec > > > > > [ 27.355670] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 937875 usec > > > > > [ 28.979671] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1006985 usec > > > > > [ 30.531674] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 941088 usec > > > > > > > > > > # the patch 16K alloc > > > > > [ 44.343380] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2296849 usec > > > > > [ 47.171290] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2014678 usec > > > > > [ 50.007258] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2094184 usec > > > > > [ 52.651141] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1953046 usec > > > > > [ 55.455089] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2209423 usec > > > > > [ 57.943153] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1941747 usec > > > > > [ 60.799043] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2038504 usec > > > > > [ 63.299007] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1788588 usec > > > > > [ 65.843011] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2137055 usec > > > > > [ 68.647031] Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2193022 usec > > > > > > > > > > 2X slower. > > > > > > > > > > perf-cycles, same test but on 64 CPUs: > > > > > > > > > > + 97.02% 0.13% [test_vmalloc] [k] fix_size_alloc_test > > > > > - 82.11% 82.10% [kernel] [k] native_queued_spin_lock_slowpath > > > > > 26.19% ret_from_fork_asm > > > > > ret_from_fork > > > > > - kthread > > > > > - 25.96% test_func > > > > > - fix_size_alloc_test > > > > > - 23.49% __vmalloc_node_noprof > > > > > - __vmalloc_node_range_noprof > > > > > - 54.70% alloc_pages_noprof > > > > > alloc_pages_mpol > > > > > __alloc_frozen_pages_noprof > > > > > get_page_from_freelist > > > > > __rmqueue_pcplist > > > > > - 5.58% __get_vm_area_node > > > > > alloc_vmap_area > > > > > - 20.54% vfree.part.0 > > > > > - 20.43% __free_frozen_pages > > > > > free_frozen_page_commit > > > > > free_pcppages_bulk > > > > > _raw_spin_lock_irqsave > > > > > native_queued_spin_lock_slowpath > > > > > - 0.77% worker_thread > > > > > - process_one_work > > > > > - 0.76% vmstat_update > > > > > refresh_cpu_vm_stats > > > > > decay_pcp_high > > > > > free_pcppages_bulk > > > > > _raw_spin_lock_irqsave > > > > > native_queued_spin_lock_slowpath > > > > > + 76.57% 0.16% [kernel] [k] _raw_spin_lock_irqsave > > > > > + 71.62% 0.00% [kernel] [k] __vmalloc_node_noprof > > > > > + 71.61% 0.58% [kernel] [k] __vmalloc_node_range_noprof > > > > > + 62.35% 0.06% [kernel] [k] alloc_pages_mpol > > > > > + 62.27% 0.17% [kernel] [k] __alloc_frozen_pages_noprof > > > > > + 62.20% 0.02% [kernel] [k] alloc_pages_noprof > > > > > + 62.10% 0.05% [kernel] [k] get_page_from_freelist > > > > > + 55.63% 0.19% [kernel] [k] __rmqueue_pcplist > > > > > + 32.11% 0.00% [kernel] [k] ret_from_fork_asm > > > > > + 32.11% 0.00% [kernel] [k] ret_from_fork > > > > > + 32.11% 0.00% [kernel] [k] kthread > > > > > > > > > > I would say the bottle-neck is a page-allocator. It seems high-order > > > > > allocations are not good for it. > > > > > > Ah also just took a closer look at this. I realize that you also did 16k > > > allocations (which is at most order-2), so it may not be a good > > > representation of high-order allocations either. > > > > > I agree. But then we should not optimize "small" orders and focus on > > highest ones. Because of double degrade. I assume stress-ng fork test > > would alos notice this. > > > > > Plus that falls into the regression range I found that I detailed in > > > response to Matthew elsewhere (I've copy pasted it here for reference) > > > > > > I ended up finding that allocating sizes <=20k had noticeable > > > regressions, while [20k, 90k] was approximately the same, and >= 90k had > > > improvements (getting more and more noticeable as size grows in > > > magnitude). > > > > > Yes, i did 2-order allocations > > > > # default > > + 35.87% 4.24% [kernel] [k] alloc_pages_bulk_noprof > > + 31.94% 0.88% [kernel] [k] vfree.part.0 > > - 27.38% 27.36% [kernel] [k] clear_page_rep > > 27.36% ret_from_fork_asm > > ret_from_fork > > kthread > > test_func > > fix_size_alloc_test > > __vmalloc_node_noprof > > __vmalloc_node_range_noprof > > alloc_pages_bulk_noprof > > clear_page_rep > > > > # patch > > + 53.32% 1.12% [kernel] [k] get_page_from_freelist > > + 49.41% 0.71% [kernel] [k] prep_new_page > > - 48.70% 48.64% [kernel] [k] clear_page_rep > > 48.64% ret_from_fork_asm > > ret_from_fork > > kthread > > test_func > > fix_size_alloc_test > > __vmalloc_node_noprof > > __vmalloc_node_range_noprof > > alloc_pages_noprof > > alloc_pages_mpol > > __alloc_frozen_pages_noprof > > get_page_from_freelist > > prep_new_page > > clear_page_rep > > > > i noticed it is because of clear_page_rep() which with patch consumes > > double in cycles. > > > > Both versions should mostly go over pcp-cache, as far as i remember > > order-2 is allowed to be cached. > > > > I wounder why the patch gives x2 of cycles to clear_page_rep()... > > > And here we go with some results "without" pcp exxecise: > > static int fix_size_alloc_test(void) > { > void **ptr; > int i; > > if (set_cpus_allowed_ptr(current, cpumask_of(1)) < 0) > pr_err("Failed to set affinity to %d CPU\n", 1); > > ptr = vmalloc(sizeof(void *) * test_loop_count); > if (!ptr) > return -1; > > for (i = 0; i < test_loop_count; i++) > ptr[i] = vmalloc((nr_pages > 0 ? nr_pages:1) * PAGE_SIZE); > > for (i = 0; i < test_loop_count; i++) { > if (ptr[i]) > vfree(ptr[i]); > } > > return 0; > } > > time sudo ./test_vmalloc.sh run_test_mask=1 nr_threads=1 nr_pages=nr-pages-in-order > > # default order-1 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1423862 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1453518 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1451734 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1455142 usec > > # patch order-1 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1431082 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1454855 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1476372 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 1433379 usec > > # default order-2 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2198130 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2208504 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2219533 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2214151 usec > > # patch order-2 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2110344 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2044186 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2083308 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2073572 usec > > # default order-3 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3718592 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3740495 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3737213 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3740765 usec > > # patch order-3 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3350391 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3374568 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3286374 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 3261335 usec > > # default order-6(64 pages) > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 23847773 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 24015706 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 24226268 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 24078102 usec > > # patch order-6 > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 20128225 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 19968964 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 20067469 usec > Summary: fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 19928870 usec > > Now i see that results align with my initial thoughts when i first time > saw your patch. Its reassuring that your test results show similar performance even at the order-1 and order-2 cases. This was what I was expecting as well. I'm assuming this happened because you tested exactly aligned PAGE_SIZE allocations (whereas somehow I hadn't thought to do that). > The question which is not clear for me still, why pcp case is doing better > even for cached orders. > > Do you have any thoughts? I'm not sure either. I'm not familiar with the optimization differences between the standard and bulk allocators :( When looking at the code, it appears that although the pcp lists can cache up to PAGE_ALLOC_COSTLY_ORDER (3), the bulk allocator doesn't have support for anything outside of order-0. And whenever order-0 pages are available, the bulk allocator appears incredibly efficient at grabbing them.