Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> On Thu, Mar 07, 2019 at 08:56:32AM -0800, Greg Thelen wrote:
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3880,6 +3880,7 @@ struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
>> * @pheadroom: out parameter for number of allocatable pages according to memcg
>> * @pdirty: out parameter for number of dirty pages
>> * @pwriteback: out parameter for number of pages under writeback
>> + * @exact: determines exact counters are required, indicates more work.
>> *
>> * Determine the numbers of file, headroom, dirty, and writeback pages in
>> * @wb's memcg. File, dirty and writeback are self-explanatory. Headroom
>> @@ -3890,18 +3891,29 @@ struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
>> * ancestors. Note that this doesn't consider the actual amount of
>> * available memory in the system. The caller should further cap
>> * *@pheadroom accordingly.
>> + *
>> + * Return value is the error precision associated with *@pdirty
>> + * and *@pwriteback. When @exact is set this a minimal value.
>> */
>> -void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>> - unsigned long *pheadroom, unsigned long *pdirty,
>> - unsigned long *pwriteback)
>> +unsigned long
>> +mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>> + unsigned long *pheadroom, unsigned long *pdirty,
>> + unsigned long *pwriteback, bool exact)
>> {
>> struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
>> struct mem_cgroup *parent;
>> + unsigned long precision;
>>
>> - *pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
>> -
>> + if (exact) {
>> + precision = 0;
>> + *pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
>> + *pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
>> + } else {
>> + precision = MEMCG_CHARGE_BATCH * num_online_cpus();
>> + *pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
>> + *pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
>> + }
>> /* this should eventually include NR_UNSTABLE_NFS */
>> - *pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
>> *pfilepages = mem_cgroup_nr_lru_pages(memcg, (1 << LRU_INACTIVE_FILE) |
>> (1 << LRU_ACTIVE_FILE));
>> *pheadroom = PAGE_COUNTER_MAX;
>> @@ -3913,6 +3925,8 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>> *pheadroom = min(*pheadroom, ceiling - min(ceiling, used));
>> memcg = parent;
>> }
>> +
>> + return precision;
>
> Have you considered unconditionally using the exact version here?
>
> It does for_each_online_cpu(), but until very, very recently we did
> this per default for all stats, for years. It only became a problem in
> conjunction with the for_each_memcg loops when frequently reading
> memory stats at the top of a very large hierarchy.
>
> balance_dirty_pages() is called against memcgs that actually own the
> inodes/memory and doesn't do the additional recursive tree collection.
>
> It's also not *that* hot of a function, and in the io path...
>
> It would simplify this patch immensely.
Good idea. Done in -v2 of the patch:
https://lore.kernel.org/lkml/20190329174609.164344-1-gthelen@xxxxxxxxxx/
