linux-kernel - Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order

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Message-ID: <1329f4ad-5fe1-41e8-97f4-0b58caf86fce@arm.com>
Date: Wed, 17 Dec 2025 17:19:13 +0530
From: Dev Jain <dev.jain@....com>
To: Uladzislau Rezki <urezki@...il.com>, Baoquan He <bhe@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Vishal Moola <vishal.moola@...il.com>, Ryan Roberts <ryan.roberts@....com>,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order_alloc parameter


On 17/12/25 5:14 pm, Uladzislau Rezki wrote:
> On Wed, Dec 17, 2025 at 11:54:26AM +0800, Baoquan He wrote:
>> Hi Uladzislau,
>>
>> On 12/16/25 at 10:19pm, Uladzislau Rezki (Sony) wrote:
>>> Introduce a module parameter to enable or disable the large-order
>>> allocation path in vmalloc. High-order allocations are disabled by
>>> default so far, but users may explicitly enable them at runtime if
>>> desired.
>>>
>>> High-order pages allocated for vmalloc are immediately split into
>>> order-0 pages and later freed as order-0, which means they do not
>>> feed the per-CPU page caches. As a result, high-order attempts tend
>> I don't get why order-0 do not feed the PCP caches.
>>
> "they" -> high-order pages. I should improve it.
>
>>> to bypass the PCP fastpath and fall back to the buddy allocator that
>>> can affect performance.
>>>
>>> However, when the PCP caches are empty, high-order allocations may
>>> show better performance characteristics especially for larger
>>> allocation requests.
>> And when PCP is empty, high-order alloc show better performance. Could
>> you please help elaborate a little more about them? Thanks.
>>
> This is what i/we measured. See below example:
>
> # 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
>
> why higher-order wins, i think it is less cyclesto get one big chunk from the
> buddy instead of looping and pick one by one.

I have the same observation that getting a higher-order chunk is faster than bulk allocating basepages.
(btw, I had resent my RFC, in case you missed!)

>
> --
> Uladzislau Rezki