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Message-ID: <aPJ6lLf24TfW_1n7@milan>
Date: Fri, 17 Oct 2025 19:19:16 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
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 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.
The question which is not clear for me still, why pcp case is doing better
even for cached orders.
Do you have any thoughts?
--
Uladzislau Rezki
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