linux-kernel - RE: [PATCH v2] percpu-internal/pcpu_chunk: Re-layout pcpu

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Message-ID: <SN7PR11MB7590D5959AF7179137A314FEE153A@SN7PR11MB7590.namprd11.prod.outlook.com>
Date:   Wed, 7 Jun 2023 15:02:32 +0000
From:   "Ma, Yu" <yu.ma@...el.com>
To:     "Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>,
        Dennis Zhou <dennis@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Zhu, Lipeng" <lipeng.zhu@...el.com>,
        "Deng, Pan" <pan.deng@...el.com>,
        "shakeelb@...gle.com" <shakeelb@...gle.com>,
        "Li, Tianyou" <tianyou.li@...el.com>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>
Subject: RE: [PATCH v2] percpu-internal/pcpu_chunk: Re-layout pcpu_chunk
 structure to reduce false sharing

Thanks Liam and Dennis for review, this is updated patch with comment around:

> 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@...ux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@...el.com>
> ---
>  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