From: Dennis Zhou <dennis@kernel.org>
To: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Yu Ma <yu.ma@intel.com>,
akpm@linux-foundation.org, tim.c.chen@intel.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
dave.hansen@intel.com, dan.j.williams@intel.com,
shakeelb@google.com, pan.deng@intel.com, tianyou.li@intel.com,
lipeng.zhu@intel.com, tim.c.chen@linux.intel.com
Subject: Re: [PATCH] percpu-internal/pcpu_chunk: Re-layout pcpu_chunk structure to reduce false sharing
Date: Tue, 6 Jun 2023 14:25:56 -0700 [thread overview]
Message-ID: <ZH-kZA5kRmWjcrTI@V92F7Y9K0C.corp.robot.car> (raw)
In-Reply-To: <20230606192127.w66indpq74ec6eyt@revolver>
Hello,
On Tue, Jun 06, 2023 at 03:21:27PM -0400, Liam R. Howlett wrote:
> * Yu Ma <yu.ma@intel.com> [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@linux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@intel.com>
> > ---
> > 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
> >
>
next prev parent reply other threads:[~2023-06-06 21:26 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-06 12:54 [PATCH] percpu-internal/pcpu_chunk: Re-layout pcpu_chunk structure to reduce false sharing Yu Ma
2023-06-06 19:21 ` Liam R. Howlett
2023-06-06 21:25 ` Dennis Zhou [this message]
2023-06-07 12:50 ` Ma, Yu
2023-06-07 14:50 ` [PATCH v2] " Yu Ma
2023-06-07 15:02 ` Ma, Yu
2023-06-09 18:20 ` Dennis Zhou
2023-06-10 0:12 ` Ma, Yu
2023-06-10 3:07 ` [PATCH v3] " Yu Ma
2023-06-12 21:43 ` Andrew Morton
2023-06-12 21:55 ` Dennis Zhou
2023-06-13 17:41 ` Ma, Yu
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