Re: [PATCH v3 11/11] memcg: check memcg dirty limits in page writeback

Greg Thelen
2010-10-20T04:34:13Z

KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:

> On Tue, 19 Oct 2010 10:00:15 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
>> On Mon, 18 Oct 2010 17:39:44 -0700
>> Greg Thelen <gthelen@google.com> wrote:
>>
>> > If the current process is in a non-root memcg, then
>> > global_dirty_limits() will consider the memcg dirty limit.
>> > This allows different cgroups to have distinct dirty limits
>> > which trigger direct and background writeback at different
>> > levels.
>> >
>> > Signed-off-by: Andrea Righi <arighi@develer.com>
>> > Signed-off-by: Greg Thelen <gthelen@google.com>
>>
>> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>
> Why FREEPAGES in memcg is not counted as dirtyable ?
>
> Thanks,
> -Kame

I think that FREEPAGES is considered dirtyable. Below I include the
latest version of the code, which includes an improved version of
memcg_hierarchical_free_pages().

Notice that mem_cgroup_page_stat(MEMCG_NR_DIRTYABLE_PAGES) returns the
sum of:
1. compute free pages using memcg_hierarchical_free_pages()
2. mem_cgroup_read_stat(mem, LRU_ACTIVE_FILE) +
mem_cgroup_read_stat(mem, LRU_INACTIVE_FILE)
3. if (mem_cgroup_can_swap(mem))
ret += mem_cgroup_read_stat(mem, LRU_ACTIVE_ANON) +
mem_cgroup_read_stat(mem, LRU_INACTIVE_ANON)

This algorithm is similar to how global (non-memcg) limits are computed
in global_dirtyable_memory().

/*
* Return the number of the number of pages that the @mem cgroup could allocate.
* If use_hierarchy is set, then this involves parent mem cgroups to find the
* cgroup with the smallest free space.
*/
static unsigned long long
memcg_hierarchical_free_pages(struct mem_cgroup *mem)
{
unsigned long free, min_free;

min_free = global_page_state(NR_FREE_PAGES) << PAGE_SHIFT;

while (mem) {
free = res_counter_read_u64(&mem->res, RES_LIMIT) -
res_counter_read_u64(&mem->res, RES_USAGE);
min_free = min(min_free, free);
mem = parent_mem_cgroup(mem);
}

/* Translate free memory in pages */
return min_free >> PAGE_SHIFT;
}

/*
* mem_cgroup_page_stat() - get memory cgroup file cache statistics
* @item: memory statistic item exported to the kernel
*
* Return the accounted statistic value.
*/
s64 mem_cgroup_page_stat(enum mem_cgroup_nr_pages_item item)
{
struct mem_cgroup *mem;
struct mem_cgroup *iter;
s64 value;

get_online_cpus();
rcu_read_lock();
mem = mem_cgroup_from_task(current);
if (mem && !mem_cgroup_is_root(mem)) {
/*
* If we're looking for dirtyable pages we need to evaluate
* free pages depending on the limit and usage of the parents
* first of all.
*/
if (item == MEMCG_NR_DIRTYABLE_PAGES)
value = memcg_hierarchical_free_pages(mem);
else
value = 0;
/*
* Recursively evaluate page statistics against all cgroup
* under hierarchy tree
*/
for_each_mem_cgroup_tree(iter, mem)
value += mem_cgroup_local_page_stat(iter, item);
} else
value = -EINVAL;
rcu_read_unlock();
put_online_cpus();

return value;
}