Re: [PATCH v3] percpu-internal/pcpu_chunk: Re-layout pcpu_chunk structure to reduce false sharing

[Andrew Morton]

On Fri,  9 Jun 2023 23:07:30 -0400 Yu Ma <yu.ma@xxxxxxxxx> wrote:

> 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%.

Well that's nice.

>
> ...
>
> --- a/mm/percpu-internal.h
> +++ b/mm/percpu-internal.h
> @@ -41,10 +41,17 @@ 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;
> -	void			*base_addr;	/* base address of this chunk */
> +	unsigned long		*bound_map;	/* boundary map */
> +	
> +	/* 
> +	 * base_addr is the base address of this chunk.
> +	 * 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 ____cacheline_aligned_in_smp;
>  
>  	unsigned long		*alloc_map;	/* allocation map */
> -	unsigned long		*bound_map;	/* boundary map */
>  	struct pcpu_block_md	*md_blocks;	/* metadata blocks */
>  
>  	void			*data;		/* chunk data */

This will of course consume more memory.  Do we have a feel for the
worst-case impact of this?