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

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Message-ID: <aUKXs_3bxLyMvTgu@milan>
Date: Wed, 17 Dec 2025 12:44:51 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vishal Moola <vishal.moola@...il.com>,
	Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order_alloc parameter

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.

--
Uladzislau Rezki