linux-kernel - Re: [RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

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Date: Wed, 24 Apr 2024 12:29:25 +0800
From: "zhangpeng (AS)" <zhangpeng362@...wei.com>
To: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
CC: <akpm@...ux-foundation.org>, <dennisszhou@...il.com>,
	<shakeelb@...gle.com>, <jack@...e.cz>, <surenb@...gle.com>,
	<kent.overstreet@...ux.dev>, <mhocko@...e.cz>, <vbabka@...e.cz>,
	<yuzhao@...gle.com>, <yu.ma@...el.com>, <wangkefeng.wang@...wei.com>,
	<sunnanyong@...wei.com>
Subject: Re: [RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

On 2024/4/18 22:20, Peng Zhang wrote:

Any suggestions or opinions are welcome. Could someone please review
this patch series?
Thanks!

> From: ZhangPeng <zhangpeng362@...wei.com>
>
> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> percpu_counter"), the rss_stats have converted into percpu_counter,
> which convert the error margin from (nr_threads * 64) to approximately
> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> performance regression on fork/exec/shell. Even after commit 14ef95be6f55
> ("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
> the performance of fork/exec/shell is still poor compared to previous
> kernel versions.
>
> To mitigate performance regression, we delay the allocation of percpu
> memory for rss_stats. Therefore, we convert mm's rss stats to use
> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> atomic mode, which reduces the memory consumption and performance
> regression caused by using percpu. For multiple-thread processes,
> rss_stat is switched to the percpu mode to reduce the error margin.
> We convert rss_stats from atomic mode to percpu mode only when the
> second thread is created.
>
> After lmbench test, we can get 2% ~ 4% performance improvement
> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
> improvement for lmbench page_fault (before batch mode[1]).
>
> The test results are as follows:
>               base           base+revert        base+this patch
>
> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
>
> [1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>
> ChangeLog:
> v2->v1:
> - Convert rss_stats from atomic mode to percpu mode only when
>    the second thread is created per Jan Kara.
> - Compared with v1, the performance data may be different due to
>    different test machines.
>
> ZhangPeng (2):
>    percpu_counter: introduce atomic mode for percpu_counter
>    mm: convert mm's rss stats to use atomic mode
>
>   include/linux/mm.h             | 50 +++++++++++++++++++++++++++++-----
>   include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++--
>   include/trace/events/kmem.h    |  4 +--
>   kernel/fork.c                  | 18 +++++++-----
>   lib/percpu_counter.c           | 31 +++++++++++++++++++--
>   5 files changed, 125 insertions(+), 21 deletions(-)
>
-- 
Best Regards,
Peng