Hello,
On Tue, Jun 06, 2023 at 03:21:27PM -0400, Liam R. Howlett wrote:
> * Yu Ma <yu.ma@xxxxxxxxx> [230606 08:27]:
> > 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%.
>
> Can we have a comment somewhere around this structure to avoid someone
> reverting this change by accident?
>
I agree with Liam. It was only recently percpu was added to the
mm_struct so this wasn't originally on the hot path. It's probably worth
reshuffling around pcpu_chunk because as you point out base_addr is
read_only after init. There in general aren't that many of these structs
on any particular host, so its probably good to just annotate with
____cacheline_aligned_in_smp and potentially reshuffle around a few
other variables.
Another optimization here is a batch allocation which hasn't been done
yet (allocate essentially an array of percpu variables all at once, but
allow for their lifetimes to be independent).
PS - I know I'm not super active, but please cc me on percpu changes.
Thanks,
Dennis
> >
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> > ---
> > mm/percpu-internal.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h
> > index f9847c131998..981eeb2ad0a9 100644
> > --- a/mm/percpu-internal.h
> > +++ b/mm/percpu-internal.h
> > @@ -41,10 +41,10 @@ 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 */
> > 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
> >
>
