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

[Dennis Zhou]

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
> > 
>