Re: [patch v4] mm: vmstat_kunit: add synthetic benchmark for vm stats

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/18/26 22:39, David Rientjes wrote:
> From: Christoph Lameter <cl@gentwo.org>
> 
> Add a synthetic benchmark that can be used to measure performance of VM
> statistics.  This is used to analyze any improvements or regressions in
> functions that are frequently used in hot code paths.
> 
> The test is run by KUnit or doing modprobe vmstat_kunit directly.
> 
> Sample output:
>  KTAP version 1
>  1..1
>  KTAP version 1
>  # Subtest: vmstat
>  # module: vmstat_kunit
>       1..3
>       # vmstat_test_inc_dec_zone_page_state: 10000 ops: inc_zone_page_state -> 8 cycles (4 ns/op), dec_zone_page_state -> 9 cycles (4 ns/op)
>       ok 1 vmstat_test_inc_dec_zone_page_state
>       # vmstat_test_interleaved_zone_page_state: 10000 ops: inc/dec pair -> 17 cycles (8 ns/op)
>       ok 2 vmstat_test_interleaved_zone_page_state
>       # vmstat_test_count_vm_event: 10000 ops: count_vm_event -> 4 cycles (2 ns/op)
>       ok 3 vmstat_test_count_vm_event
>  # vmstat: pass:3 fail:0 skip:0 total:3
>  # Totals: pass:3 fail:0 skip:0 total:3
>  ok 1 vmstat
> 
> Assisted-by: Gemini:gemini-3.8-flash
> Signed-off-by: Christoph Lameter <cl@gentwo.org>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> v4:
>  - included sample output in the commit description
>  - switched to NR_MLOCK which is display only so no side effects
>  - remove unnecessary "rem" variable
> 
> Once we're happy with this change, I'll apply the same treatment to the
> proposed pgalloc and slab tests.
> 
>  MAINTAINERS             |   1 +
>  mm/Kconfig              |  11 +++
>  mm/Makefile             |   1 +
>  mm/tests/vmstat_kunit.c | 157 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 170 insertions(+)
>  create mode 100644 mm/tests/vmstat_kunit.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 3b2eb2a7a89a..de09076d91b9 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -17161,6 +17161,7 @@ F:	mm/ptdump.c
>  F:	mm/sparse-vmemmap.c
>  F:	mm/sparse.c
>  F:	mm/sparse.h
> +F:	mm/tests/vmstat_kunit.c
>  F:	mm/util.c
>  F:	mm/vmpressure.c
>  F:	mm/vmstat.c
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 604c58199acb..2d021fb5ace6 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1511,6 +1511,17 @@ config LAZY_MMU_MODE_KUNIT_TEST
>  
>  	  If unsure, say N.
>  
> +config VMSTAT_KUNIT_TEST
> +	tristate "KUnit test for VM statistics" if !KUNIT_ALL_TESTS
> +	depends on KUNIT
> +	default KUNIT_ALL_TESTS
> +	help
> +	  Enable this option to test and benchmark the performance of VM
> +	  statistics updates (zone page state and VM event counters), which
> +	  are used frequently in hot memory management code paths.
> +
> +	  If unsure, say N.
> +
>  source "mm/damon/Kconfig"
>  
>  endmenu
> diff --git a/mm/Makefile b/mm/Makefile
> index e7245cb88c66..3d4f2c43b8d3 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -147,4 +147,5 @@ obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
>  obj-$(CONFIG_EXECMEM) += execmem.o
>  obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
>  obj-$(CONFIG_LAZY_MMU_MODE_KUNIT_TEST) += tests/lazy_mmu_mode_kunit.o
> +obj-$(CONFIG_VMSTAT_KUNIT_TEST) += tests/vmstat_kunit.o
>  obj-$(CONFIG_MEM_ALLOC_PROFILING) += alloc_tag.o
> diff --git a/mm/tests/vmstat_kunit.c b/mm/tests/vmstat_kunit.c
> new file mode 100644
> index 000000000000..899c957091d1
> --- /dev/null
> +++ b/mm/tests/vmstat_kunit.c
> @@ -0,0 +1,157 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * KUnit synthetic performance benchmark for VM statistics.
> + *
> + * (C) 2009 Linux Foundation, Christoph Lameter <cl@gentwo.org>
> + * (C) 2026 Google LLC, David Rientjes <rientjes@google.com>
> + */
> +#include <kunit/test.h>
> +#include <linux/mm.h>
> +#include <linux/vmstat.h>
> +#include <linux/timex.h>
> +#include <linux/ktime.h>
> +#include <linux/math64.h>
> +
> +#define TEST_COUNT 10000
> +
> +static void vmstat_test_free_page(void *arg)
> +{
> +	__free_page((struct page *)arg);
> +}
> +
> +/*
> + * Test 1: Sequential inc_zone_page_state() followed by dec_zone_page_state().
> + * Net change to zone counters is 0.
> + */
> +static void vmstat_test_inc_dec_zone_page_state(struct kunit *test)
> +{
> +	struct page *page;
> +	cycles_t time1, time2, time;
> +	u64 t1_ns, t2_ns;
> +	u64 inc_cycles, dec_cycles;
> +	u64 inc_ns, dec_ns;
> +	unsigned int i;
> +
> +	page = alloc_page(GFP_KERNEL);
> +	KUNIT_ASSERT_NOT_NULL(test, page);
> +	KUNIT_ASSERT_EQ(test, kunit_add_action_or_reset(test, vmstat_test_free_page, page), 0);
> +
> +	/* Benchmark inc_zone_page_state() */
> +	time1 = get_cycles();
> +	t1_ns = ktime_get_ns();
> +	for (i = 0; i < TEST_COUNT; i++)
> +		inc_zone_page_state(page, NR_MLOCK);

Just curious, could some compiler optimizations here change the picture, and
doing the average would not actually be representative? Not sure if a barrier()
would reliably avoid that.

Maybe not relevant today, but I do wonder if we should take care that it stays
that way.

Applies to all cases below where we loop.
This looks much cleaner to me, thanks!

-- 
Cheers,

David