On Mon, Jan 28, 2013 at 10:10 PM, Michal Hocko <mhocko@xxxxxxx> wrote:
> Hi,
> just a minor comment/suggestion. The patch would be much more easier to
> review if you split it up into two parts. Preparatory with page->memcg
> parameter change and the locking change you are proposing.
>
OK.
> On Sat 26-01-13 19:12:36, Sha Zhengju wrote:
> [...]
>> So in order to make the lock simpler and clearer and also avoid the 'nesting'
>> problem, a choice may be:
>> (CPU-A does "page stat accounting" and CPU-B does "move")
>>
>> CPU-A CPU-B
>>
>> move_lock_mem_cgroup()
>> memcg = pc->mem_cgroup
>> TestSetPageDirty(page)
>> move_unlock_mem_cgroup()
>> move_lock_mem_cgroup()
>> if (PageDirty) {
>> old_memcg->nr_dirty --;
>> new_memcg->nr_dirty ++;
>> }
>> pc->mem_cgroup = new_memcg
>> move_unlock_mem_cgroup()
>>
>> memcg->nr_dirty ++
>>
>> For CPU-A, we save pc->mem_cgroup in a temporary variable just before
>> TestSetPageDirty inside move_lock and then update stats if the page is set
>> PG_dirty successfully.
>
> Hmm, the description is a bit confising. You are talking about
> TestSetPageDirty but dirty accounting is not directly handled in the
> patch. It took me a bit to figure that it's actually set_page_dirty
> called from page_remove_rmap which matters here. So it is more a
Thanks for reviewing!
Yeah, now I find it improper to take dirty pages as example in commit
log. Since dirty page accounting is more complicate than other stats
such as FILE_MAPPED and FILE_WRITEBACK, I tried to clear out the
reason of changing the current lock rule.
> dependency between MEMCG_NR_FILE_MAPPED and your future (currently
> non-existent) MEMCG_NR_FILE_DIRTY accounting that you are preparing.
> set_page_dirty now can take mem_cgroup_{begin,end}_update_page_stat.
>
>> But CPU-B may do "moving" in advance that
>> "old_memcg->nr_dirty --" will make old_memcg->nr_dirty incorrect but
>> soon CPU-A will do "memcg->nr_dirty ++" finally that amend the stats.
>
> The counter is per-cpu so we are safe wrt. atomic increments and we can
> probably tolerate off-by 1 temporal errors (mem_cgroup_read_stat would
> need val = min(val, 0);).
Sorry, I cannot catch the 'min(val, 0)' part.. or do you mean max?
> I am not sure I like this very much though. It adds an additional memcg
> reference counting into page_add_file which is a hot path already.
>
> I think the accounting side should be as lightweight as possible and the
> additional price should be payed by mover.
>
Yes... I thought css_get is not so heavy here so I used the existing
try_get(). I'll try to use rcu to hold memcg alive as Kame suggest.
>> Signed-off-by: Sha Zhengju <handai.szj@xxxxxxxxxx>
>> ---
>> include/linux/memcontrol.h | 14 +++++------
>> mm/memcontrol.c | 8 ++-----
>> mm/rmap.c | 55 +++++++++++++++++++++++++++++++++-----------
>> 3 files changed, 51 insertions(+), 26 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 0108a56..12de53b 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -164,20 +164,20 @@ static inline void mem_cgroup_end_update_page_stat(struct page *page,
>> rcu_read_unlock();
>> }
>>
>> -void mem_cgroup_update_page_stat(struct page *page,
>> +void mem_cgroup_update_page_stat(struct mem_cgroup *memcg,
>> enum mem_cgroup_page_stat_item idx,
>> int val);
>>
>> -static inline void mem_cgroup_inc_page_stat(struct page *page,
>> +static inline void mem_cgroup_inc_page_stat(struct mem_cgroup *memcg,
>> enum mem_cgroup_page_stat_item idx)
>> {
>> - mem_cgroup_update_page_stat(page, idx, 1);
>> + mem_cgroup_update_page_stat(memcg, idx, 1);
>> }
>>
>> -static inline void mem_cgroup_dec_page_stat(struct page *page,
>> +static inline void mem_cgroup_dec_page_stat(struct mem_cgroup *memcg,
>> enum mem_cgroup_page_stat_item idx)
>> {
>> - mem_cgroup_update_page_stat(page, idx, -1);
>> + mem_cgroup_update_page_stat(memcg, idx, -1);
>> }
>>
>> unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
>> @@ -354,12 +354,12 @@ static inline void mem_cgroup_end_update_page_stat(struct page *page,
>> {
>> }
>>
>> -static inline void mem_cgroup_inc_page_stat(struct page *page,
>> +static inline void mem_cgroup_inc_page_stat(struct mem_cgroup *memcg,
>> enum mem_cgroup_page_stat_item idx)
>> {
>> }
>>
>> -static inline void mem_cgroup_dec_page_stat(struct page *page,
>> +static inline void mem_cgroup_dec_page_stat(struct mem_cgroup *memcg,
>> enum mem_cgroup_page_stat_item idx)
>> {
>> }
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 3817460..1b13e43 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2259,18 +2259,14 @@ void __mem_cgroup_end_update_page_stat(struct page *page, unsigned long *flags)
>> move_unlock_mem_cgroup(pc->mem_cgroup, flags);
>> }
>>
>> -void mem_cgroup_update_page_stat(struct page *page,
>> +void mem_cgroup_update_page_stat(struct mem_cgroup *memcg,
>> enum mem_cgroup_page_stat_item idx, int val)
>> {
>> - struct mem_cgroup *memcg;
>> - struct page_cgroup *pc = lookup_page_cgroup(page);
>> - unsigned long uninitialized_var(flags);
>>
>> if (mem_cgroup_disabled())
>> return;
>>
>> - memcg = pc->mem_cgroup;
>> - if (unlikely(!memcg || !PageCgroupUsed(pc)))
>> + if (unlikely(!memcg))
>> return;
>>
>> switch (idx) {
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 59b0dca..0d74c48 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1112,13 +1112,25 @@ void page_add_file_rmap(struct page *page)
>> {
>> bool locked;
>> unsigned long flags;
>> + bool ret;
>> + struct mem_cgroup *memcg = NULL;
>> + struct cgroup_subsys_state *css = NULL;
>>
>> mem_cgroup_begin_update_page_stat(page, &locked, &flags);
>> - if (atomic_inc_and_test(&page->_mapcount)) {
>> + memcg = try_get_mem_cgroup_from_page(page);
>> + ret = atomic_inc_and_test(&page->_mapcount);
>> + mem_cgroup_end_update_page_stat(page, &locked, &flags);
>> +
>> + if (ret) {
>> __inc_zone_page_state(page, NR_FILE_MAPPED);
>> - mem_cgroup_inc_page_stat(page, MEMCG_NR_FILE_MAPPED);
>> + if (memcg)
>> + mem_cgroup_inc_page_stat(memcg, MEMCG_NR_FILE_MAPPED);
>> + }
>> +
>> + if (memcg) {
>> + css = mem_cgroup_css(memcg);
>> + css_put(css);
>> }
>> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
>> }
>>
>> /**
>> @@ -1133,18 +1145,32 @@ void page_remove_rmap(struct page *page)
>> bool anon = PageAnon(page);
>> bool locked;
>> unsigned long flags;
>> + struct mem_cgroup *memcg = NULL;
>> + struct cgroup_subsys_state *css = NULL;
>> + bool ret;
>>
>> /*
>> * The anon case has no mem_cgroup page_stat to update; but may
>> * uncharge_page() below, where the lock ordering can deadlock if
>> * we hold the lock against page_stat move: so avoid it on anon.
>> */
>> - if (!anon)
>> + if (!anon) {
>> mem_cgroup_begin_update_page_stat(page, &locked, &flags);
>> + memcg = try_get_mem_cgroup_from_page(page);
>> + if (memcg)
>> + css = mem_cgroup_css(memcg);
>> + }
>> +
>> + ret = atomic_add_negative(-1, &page->_mapcount);
>> + if (!anon)
>> + mem_cgroup_end_update_page_stat(page, &locked, &flags);
>>
>> /* page still mapped by someone else? */
>> - if (!atomic_add_negative(-1, &page->_mapcount))
>> - goto out;
>> + if (!ret) {
>> + if (!anon && memcg)
>> + css_put(css);
>> + return;
>> + }
>>
>> /*
>> * Now that the last pte has gone, s390 must transfer dirty
>> @@ -1173,8 +1199,12 @@ void page_remove_rmap(struct page *page)
>> * Hugepages are not counted in NR_ANON_PAGES nor NR_FILE_MAPPED
>> * and not charged by memcg for now.
>> */
>> - if (unlikely(PageHuge(page)))
>> - goto out;
>> + if (unlikely(PageHuge(page))) {
>> + if (!anon && memcg)
>> + css_put(css);
>> + return;
>> + }
>> +
>> if (anon) {
>> mem_cgroup_uncharge_page(page);
>> if (!PageTransHuge(page))
>> @@ -1184,8 +1214,10 @@ void page_remove_rmap(struct page *page)
>> NR_ANON_TRANSPARENT_HUGEPAGES);
>> } else {
>> __dec_zone_page_state(page, NR_FILE_MAPPED);
>> - mem_cgroup_dec_page_stat(page, MEMCG_NR_FILE_MAPPED);
>> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
>> + if (memcg) {
>> + mem_cgroup_dec_page_stat(memcg, MEMCG_NR_FILE_MAPPED);
>> + css_put(css);
>> + }
>> }
>> if (unlikely(PageMlocked(page)))
>> clear_page_mlock(page);
>> @@ -1199,9 +1231,6 @@ void page_remove_rmap(struct page *page)
>> * faster for those pages still in swapcache.
>> */
>> return;
>> -out:
>> - if (!anon)
>> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
>> }
>>
>> /*
>> --
>> 1.7.9.5
>>
>> --
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>
> --
> Michal Hocko
> SUSE Labs
--
Thanks,
Sha
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