[PATCH RESEND v2] mm/percpu-internal.h: optimise pcpu_chunk struct to save memory

[PATCH RESEND v2] mm/percpu-internal.h: optimise pcpu_chunk struct to save memory

[Hongling Zeng]

From: zenghongling <zenghongling@kylinos.cn>

Using pahole, we can see that there are some padding holes
in the current pcpu_chunk structure,Adjusting the layout of pcpu_chunk
can reduce these holes,decreasing its size from 192 bytes to 128 bytes
and eliminating a wasted cache line.

With allmodconfig (CONFIG_PERCPU_STATS + NEED_PCPUOBJ_EXT)
Before:
 /* size: 256, cachelines: 4, members: 19 */

After:
 /* size: 192, cachelines: 3, members: 19 */

with NEED_PCPUOBJ_EXT
Before:
struct pcpu_chunk {
        struct list_head           list;                 /*     0    16 */
        int                        free_bytes;           /*    16     4 */
        struct pcpu_block_md       chunk_md;             /*    20    32 */

        /* XXX 4 bytes hole, try to pack */

        long unsigned int *        bound_map;            /*    56     8 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        void *                     base_addr __attribute__((__aligned__(64))); /*    64     8 */
        long unsigned int *        alloc_map;            /*    72     8 */
        struct pcpu_block_md *     md_blocks;            /*    80     8 */
        void *                     data;                 /*    88     8 */
        bool                       immutable;            /*    96     1 */
        bool                       isolated;             /*    97     1 */

        /* XXX 2 bytes hole, try to pack */

        int                        start_offset;         /*   100     4 */
        int                        end_offset;           /*   104     4 */

        /* XXX 4 bytes hole, try to pack */

        struct obj_cgroup * *      obj_cgroups;          /*   112     8 */
        int                        nr_pages;             /*   120     4 */
        int                        nr_populated;         /*   124     4 */
        /* --- cacheline 2 boundary (128 bytes) --- */
        int                        nr_empty_pop_pages;   /*   128     4 */

        /* XXX 4 bytes hole, try to pack */

        long unsigned int          populated[];          /*   136     0 */

        /* size: 192, cachelines: 3, members: 17 */
        /* sum members: 122, holes: 4, sum holes: 14 */
        /* padding: 56 */
        /* forced alignments: 1 */
} __attribute__((__aligned__(64)));

After:
struct pcpu_chunk {
	struct list_head           list;                 /*     0    16 */
	int                        free_bytes;           /*    16     4 */
	struct pcpu_block_md       chunk_md;             /*    20    32 */

	/* XXX 4 bytes hole, try to pack */

	long unsigned int *        bound_map;            /*    56     8 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	void *                     base_addr __attribute__((__aligned__(64))); /*    64     8 */
	long unsigned int *        alloc_map;            /*    72     8 */
	struct pcpu_block_md *     md_blocks;            /*    80     8 */
	void *                     data;                 /*    88     8 */
	bool                       immutable;            /*    96     1 */
	bool                       isolated;             /*    97     1 */

	/* XXX 2 bytes hole, try to pack */

	int                        start_offset;         /*   100     4 */
	int                        end_offset;           /*   104     4 */
	int                        nr_pages;             /*   108     4 */
	int                        nr_populated;         /*   112     4 */
	int                        nr_empty_pop_pages;   /*   116     4 */
	struct obj_cgroup * *      obj_cgroups;          /*   120     8 */
	/* --- cacheline 2 boundary (128 bytes) --- */
	long unsigned int          populated[];          /*   128     0 */

	/* size: 128, cachelines: 2, members: 17 */
	/* sum members: 122, holes: 2, sum holes: 6 */
	/* forced alignments: 1 */
} __attribute__((__aligned__(64)));

Suggested-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: zenghongling <zenghongling@kylinos.cn>

---
Changes in v2:
  - Add pahole output for allmodconfig (CONFIG_PERCPU_STATS + NEED_PCPUOBJ_EXT)
  - Fix subject to use "pcpu_chuck struct"
  - Reorder nr_pages before nr_empty_pop_pages
  - Add suggest by

Changes in  v1:
  - Fix the error commit message.
  - Move nr_pages over nr_empty_pop_pages.
---
---
 mm/percpu-internal.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h
index 4b3d6ec43703..8cbe039bf847 100644
--- a/mm/percpu-internal.h
+++ b/mm/percpu-internal.h
@@ -77,13 +77,13 @@ struct pcpu_chunk {
 	int			end_offset;	/* additional area required to
 						   have the region end page
 						   aligned */
+	int			nr_pages;	/* # of pages served by this chunk */
+	int			nr_populated;	/* # of populated pages */
+	int                     nr_empty_pop_pages; /* # of empty populated pages */
 #ifdef NEED_PCPUOBJ_EXT
 	struct pcpuobj_ext	*obj_exts;	/* vector of object cgroups */
 #endif
 
-	int			nr_pages;	/* # of pages served by this chunk */
-	int			nr_populated;	/* # of populated pages */
-	int                     nr_empty_pop_pages; /* # of empty populated pages */
 	unsigned long		populated[];	/* populated bitmap */
 };
 
-- 
2.25.1

Re: [PATCH RESEND v2] mm/percpu-internal.h: optimise pcpu_chunk struct to save memory

[Andrew Morton]

On Mon, 11 May 2026 15:03:09 +0800 Hongling Zeng <zenghongling@kylinos.cn> wrote:

> Using pahole, we can see that there are some padding holes
> in the current pcpu_chunk structure,Adjusting the layout of pcpu_chunk
> can reduce these holes,decreasing its size from 192 bytes to 128 bytes
> and eliminating a wasted cache line.
> 
> With allmodconfig (CONFIG_PERCPU_STATS + NEED_PCPUOBJ_EXT)
> Before:
>  /* size: 256, cachelines: 4, members: 19 */
> 
> After:
>  /* size: 192, cachelines: 3, members: 19 */

Thanks, I'll add this to mm.git for testing, but I'm not planning on
taking it further until Dennis has taken a look.

Re: [PATCH RESEND v2] mm/percpu-internal.h: optimise pcpu_chunk struct to save memory

[Dennis Zhou]

Hi Andrew,

On Tue, May 12, 2026 at 03:03:40PM -0700, Andrew Morton wrote:
> On Mon, 11 May 2026 15:03:09 +0800 Hongling Zeng <zenghongling@kylinos.cn> wrote:
> 
> > Using pahole, we can see that there are some padding holes
> > in the current pcpu_chunk structure,Adjusting the layout of pcpu_chunk
> > can reduce these holes,decreasing its size from 192 bytes to 128 bytes
> > and eliminating a wasted cache line.
> > 
> > With allmodconfig (CONFIG_PERCPU_STATS + NEED_PCPUOBJ_EXT)
> > Before:
> >  /* size: 256, cachelines: 4, members: 19 */
> > 
> > After:
> >  /* size: 192, cachelines: 3, members: 19 */
> 
> Thanks, I'll add this to mm.git for testing, but I'm not planning on
> taking it further until Dennis has taken a look.
> 

Sorry, I thought we took care of this one prior before. This is per
chunk metadata, so it won't save much. That being said it doesn't hurt.

Acked-by: Dennis Zhou <dennis@kernel.org>

I'm thinking about the other percpu series now.

Thanks,
Dennis