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SubjectRe: [RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode
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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@huawei.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


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