内存中ｃｇｒｏｕｐ：
发现ｍｅｍｏｒｙ cgroup中mem.usage_in_bytes是通过下面这个数值得到的：
val = page_counter_read(&memcg->memory);
然后计数的增加在函数:mm/page_counter.c文件中的：page_counter_charge函数，
mem_cgroup_try_charge-->
-----> try_charge--->page_counter_charge(memcg->memory)
mem_cgroup_try_charge_skmem -->
soft_limit_excess(struct memcgroup)
446 static unsigned long soft_limit_excess(struct mem_cgroup *memcg)
447 {
448 unsigned long nr_pages = page_counter_read(&memcg->memory);
449 unsigned long soft_limit = READ_ONCE(memcg->soft_limit);
450 unsigned long excess = 0;
451
452 if (nr_pages > soft_limit)
453 excess = nr_pages - soft_limit;
454
455 return excess;
456 }
457
PATH1) do_try_to_free_pages---->shrink_zones----->
mem_cgroup_soft_limit_reclaim ---> mem_cgroup_soft_reclaim-->mem_cgroup_shrink_node
PATH2) kswapd_init----->kswaped---->balance_bgdat---->
其中ＰＡＴＨ１）有这么几条路到达：try_to_free_pages try_to_free_mem_cgroup_pages shrink_all_memory
try_to_free_mem_cgroup_pages(mm/memcontrol.c) 在try_charge/force_empty/memory_max_writｅ......等等条件的时候就会触发