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

[Ma, Yu]

> 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.
> 
Thanks Liam and Dennis for quick feedback, I'll send out the updated patch with comment around.

> 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.
> 
LOL, sure :)

> 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
> > >
> >