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

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Message-ID: <aUPYzWNHjkC0p4lX@MiWiFi-R3L-srv>
Date: Thu, 18 Dec 2025 18:34:53 +0800
From: Baoquan He <bhe@...hat.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: 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 12/17/25 at 12:44pm, 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.

Ah, git it now, thanks.

> 
> > > 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.

Thanks a lot for the details.