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
