[PATCH v2] percpu-internal/pcpu_chunk: Re-layout pcpu_chunk structure to reduce false sharing

[Yu Ma]

When running UnixBench/Execl throughput case, false sharing is observed
due to frequent read on base_addr and write on free_bytes, chunk_md.

UnixBench/Execl represents a class of workload where bash scripts
are spawned frequently to do some short jobs. It will do system call on
execl frequently, and execl will call mm_init to initialize mm_struct
of the process. mm_init will call __percpu_counter_init for
percpu_counters initialization. Then pcpu_alloc is called to read
the base_addr of pcpu_chunk for memory allocation. Inside pcpu_alloc,
it will call pcpu_alloc_area  to allocate memory from a specified chunk.
This function will update "free_bytes" and "chunk_md" to record the
rest free bytes and other meta data for this chunk. Correspondingly,
pcpu_free_area will also update these 2 members when free memory.
Call trace from perf is as below:
+   57.15%  0.01%  execl   [kernel.kallsyms] [k] __percpu_counter_init
+   57.13%  0.91%  execl   [kernel.kallsyms] [k] pcpu_alloc
-   55.27% 54.51%  execl   [kernel.kallsyms] [k] osq_lock
   - 53.54% 0x654278696e552f34
        main
        __execve
        entry_SYSCALL_64_after_hwframe
        do_syscall_64
        __x64_sys_execve
        do_execveat_common.isra.47
        alloc_bprm
        mm_init
        __percpu_counter_init
        pcpu_alloc
      - __mutex_lock.isra.17

In current pcpu_chunk layout, ‘base_addr’ is in the same cache line
with ‘free_bytes’ and ‘chunk_md’, and ‘base_addr’ is at the
last 8 bytes. This patch moves ‘bound_map’ up to ‘base_addr’,
to let ‘base_addr’ locate in a new cacheline.

With this change, on Intel Sapphire Rapids 112C/224T platform,
based on v6.4-rc4, the 160 parallel score improves by 24%.

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
---
 mm/percpu-internal.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h
index f9847c131998..ecc7be1ec876 100644
--- a/mm/percpu-internal.h
+++ b/mm/percpu-internal.h
@@ -41,10 +41,16 @@ struct pcpu_chunk {
 	struct list_head	list;		/* linked to pcpu_slot lists */
 	int			free_bytes;	/* free bytes in the chunk */
 	struct pcpu_block_md	chunk_md;
+	unsigned long		*bound_map;	/* boundary map */
+	
+	/*
+	 * To reduce false sharing, current layout is optimized to make sure
+	 * base_addr locate in the different cacheline with free_bytes and
+	 * chunk_md.
+	 */
 	void			*base_addr;	/* base address of this chunk */
 
 	unsigned long		*alloc_map;	/* allocation map */
-	unsigned long		*bound_map;	/* boundary map */
 	struct pcpu_block_md	*md_blocks;	/* metadata blocks */
 
 	void			*data;		/* chunk data */
-- 
2.39.3