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Message-ID: <aPZ-IbX7K8f4Jngp@fedora>
Date: Mon, 20 Oct 2025 11:23:29 -0700
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] mm/vmalloc: request large order pages from buddy
allocator
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.
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