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.c...
ZhangPeng (2): percpu_counter: introduce atomic mode for percpu_counter mm: convert mm's rss stats to use atomic mode
include/linux/mm.h | 61 ++++++++++++++++++++++++++++++---- include/linux/percpu_counter.h | 48 ++++++++++++++++++++++++-- include/trace/events/kmem.h | 4 +-- kernel/fork.c | 28 ++++++++++++++-- lib/percpu_counter.c | 35 +++++++++++++++++-- 5 files changed, 159 insertions(+), 17 deletions(-)