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?
