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

[Peng Zhang <zhangpeng362@huawei.com>]

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(-)

-- 
2.25.1