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SubjectRe: [PATCH 4/9] memcg: dirty page accounting support routines
On Tue, 28 Feb 2012 22:00:26 +0800
Fengguang Wu <fengguang.wu@intel.com> wrote:

> From: Greg Thelen <gthelen@google.com>
>
> Added memcg dirty page accounting support routines. These routines are
> used by later changes to provide memcg aware writeback and dirty page
> limiting. A mem_cgroup_dirty_info() tracepoint is is also included to
> allow for easier understanding of memcg writeback operation.
>
> ...
>
> +/*
> + * Return the number of additional pages that the @memcg cgroup could allocate.
> + * If use_hierarchy is set, then this involves checking parent mem cgroups to
> + * find the cgroup with the smallest free space.
> + */

Comment needs revisting - use_hierarchy does not exist.

> +static unsigned long
> +mem_cgroup_hierarchical_free_pages(struct mem_cgroup *memcg)
> +{
> + u64 free;
> + unsigned long min_free;
> +
> + min_free = global_page_state(NR_FREE_PAGES);
> +
> + while (memcg) {
> + free = mem_cgroup_margin(memcg);
> + min_free = min_t(u64, min_free, free);
> + memcg = parent_mem_cgroup(memcg);
> + }
> +
> + return min_free;
> +}
> +
> +/*
> + * mem_cgroup_page_stat() - get memory cgroup file cache statistics
> + * @memcg: memory cgroup to query
> + * @item: memory statistic item exported to the kernel
> + *
> + * Return the accounted statistic value.
> + */
> +unsigned long mem_cgroup_page_stat(struct mem_cgroup *memcg,
> + enum mem_cgroup_page_stat_item item)
> +{
> + struct mem_cgroup *iter;
> + s64 value;
> +
> + /*
> + * 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 = mem_cgroup_hierarchical_free_pages(memcg);
> + else
> + value = 0;
> +
> + /*
> + * Recursively evaluate page statistics against all cgroup under
> + * hierarchy tree
> + */
> + for_each_mem_cgroup_tree(iter, memcg)
> + value += mem_cgroup_local_page_stat(iter, item);

What's the locking rule for for_each_mem_cgroup_tree()? It's unobvious
from the code and isn't documented?

> + /*
> + * Summing of unlocked per-cpu counters is racy and may yield a slightly
> + * negative value. Zero is the only sensible value in such cases.
> + */
> + if (unlikely(value < 0))
> + value = 0;
> +
> + return value;
> +}
> +
>
> ...
>


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