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Date: Mon, 5 Jan 2015 10:51:13 +0100
From: Michal Hocko <mhocko@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: memcontrol: track move_lock state internally
On Fri 02-01-15 15:58:47, Johannes Weiner wrote:
> The complexity of memcg page stat synchronization is currently leaking
> into the callsites, forcing them to keep track of the move_lock state
> and the IRQ flags. Simplify the API by tracking it in the memcg.
OK, 16B per memcg is OK considering the trickiness stays in memcg.
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Michal Hocko <mhocko@...e.cz>
Thanks!
> ---
> include/linux/memcontrol.h | 6 ++--
> mm/memcontrol.c | 68 ++++++++++++++++++++++++++--------------------
> mm/page-writeback.c | 12 +++-----
> mm/rmap.c | 12 +++-----
> 4 files changed, 49 insertions(+), 49 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index fb212e1d700d..04d3c2028782 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -138,12 +138,10 @@ static inline bool mem_cgroup_disabled(void)
> return false;
> }
>
> -struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page, bool *locked,
> - unsigned long *flags);
> -void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool *locked,
> - unsigned long *flags);
> +struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page);
> void mem_cgroup_update_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx, int val);
> +void mem_cgroup_end_page_stat(struct mem_cgroup *memcg);
>
> static inline void mem_cgroup_inc_page_stat(struct mem_cgroup *memcg,
> enum mem_cgroup_stat_index idx)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a855848627a5..eccc0ed3b6f3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -325,9 +325,11 @@ struct mem_cgroup {
> /*
> * set > 0 if pages under this cgroup are moving to other cgroup.
> */
> - atomic_t moving_account;
> + atomic_t moving_account;
> /* taken only while moving_account > 0 */
> - spinlock_t move_lock;
> + spinlock_t move_lock;
> + struct task_struct *move_lock_task;
> + unsigned long move_lock_flags;
> /*
> * percpu counter.
> */
> @@ -1977,34 +1979,33 @@ cleanup:
> /**
> * mem_cgroup_begin_page_stat - begin a page state statistics transaction
> * @page: page that is going to change accounted state
> - * @locked: &memcg->move_lock slowpath was taken
> - * @flags: IRQ-state flags for &memcg->move_lock
> *
> * This function must mark the beginning of an accounted page state
> * change to prevent double accounting when the page is concurrently
> * being moved to another memcg:
> *
> - * memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
> + * memcg = mem_cgroup_begin_page_stat(page);
> * if (TestClearPageState(page))
> * mem_cgroup_update_page_stat(memcg, state, -1);
> - * mem_cgroup_end_page_stat(memcg, locked, flags);
> - *
> - * The RCU lock is held throughout the transaction. The fast path can
> - * get away without acquiring the memcg->move_lock (@locked is false)
> - * because page moving starts with an RCU grace period.
> - *
> - * The RCU lock also protects the memcg from being freed when the page
> - * state that is going to change is the only thing preventing the page
> - * from being uncharged. E.g. end-writeback clearing PageWriteback(),
> - * which allows migration to go ahead and uncharge the page before the
> - * account transaction might be complete.
> + * mem_cgroup_end_page_stat(memcg);
> */
> -struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page,
> - bool *locked,
> - unsigned long *flags)
> +struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page)
> {
> struct mem_cgroup *memcg;
> + unsigned long flags;
>
> + /*
> + * The RCU lock is held throughout the transaction. The fast
> + * path can get away without acquiring the memcg->move_lock
> + * because page moving starts with an RCU grace period.
> + *
> + * The RCU lock also protects the memcg from being freed when
> + * the page state that is going to change is the only thing
> + * preventing the page from being uncharged.
> + * E.g. end-writeback clearing PageWriteback(), which allows
> + * migration to go ahead and uncharge the page before the
> + * account transaction might be complete.
> + */
> rcu_read_lock();
>
> if (mem_cgroup_disabled())
> @@ -2014,16 +2015,22 @@ again:
> if (unlikely(!memcg))
> return NULL;
>
> - *locked = false;
> if (atomic_read(&memcg->moving_account) <= 0)
> return memcg;
>
> - spin_lock_irqsave(&memcg->move_lock, *flags);
> + spin_lock_irqsave(&memcg->move_lock, flags);
> if (memcg != page->mem_cgroup) {
> - spin_unlock_irqrestore(&memcg->move_lock, *flags);
> + spin_unlock_irqrestore(&memcg->move_lock, flags);
> goto again;
> }
> - *locked = true;
> +
> + /*
> + * When charge migration first begins, we can have locked and
> + * unlocked page stat updates happening concurrently. Track
> + * the task who has the lock for mem_cgroup_end_page_stat().
> + */
> + memcg->move_lock_task = current;
> + memcg->move_lock_flags = flags;
>
> return memcg;
> }
> @@ -2031,14 +2038,17 @@ again:
> /**
> * mem_cgroup_end_page_stat - finish a page state statistics transaction
> * @memcg: the memcg that was accounted against
> - * @locked: value received from mem_cgroup_begin_page_stat()
> - * @flags: value received from mem_cgroup_begin_page_stat()
> */
> -void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool *locked,
> - unsigned long *flags)
> +void mem_cgroup_end_page_stat(struct mem_cgroup *memcg)
> {
> - if (memcg && *locked)
> - spin_unlock_irqrestore(&memcg->move_lock, *flags);
> + if (memcg && memcg->move_lock_task == current) {
> + unsigned long flags = memcg->move_lock_flags;
> +
> + memcg->move_lock_task = NULL;
> + memcg->move_lock_flags = 0;
> +
> + spin_unlock_irqrestore(&memcg->move_lock, flags);
> + }
>
> rcu_read_unlock();
> }
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 6f4335238e33..fb71e9deca85 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2308,12 +2308,10 @@ EXPORT_SYMBOL(clear_page_dirty_for_io);
> int test_clear_page_writeback(struct page *page)
> {
> struct address_space *mapping = page_mapping(page);
> - unsigned long memcg_flags;
> struct mem_cgroup *memcg;
> - bool locked;
> int ret;
>
> - memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);
> + memcg = mem_cgroup_begin_page_stat(page);
> if (mapping) {
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> unsigned long flags;
> @@ -2338,19 +2336,17 @@ int test_clear_page_writeback(struct page *page)
> dec_zone_page_state(page, NR_WRITEBACK);
> inc_zone_page_state(page, NR_WRITTEN);
> }
> - mem_cgroup_end_page_stat(memcg, &locked, &memcg_flags);
> + mem_cgroup_end_page_stat(memcg);
> return ret;
> }
>
> int __test_set_page_writeback(struct page *page, bool keep_write)
> {
> struct address_space *mapping = page_mapping(page);
> - unsigned long memcg_flags;
> struct mem_cgroup *memcg;
> - bool locked;
> int ret;
>
> - memcg = mem_cgroup_begin_page_stat(page, &locked, &memcg_flags);
> + memcg = mem_cgroup_begin_page_stat(page);
> if (mapping) {
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> unsigned long flags;
> @@ -2380,7 +2376,7 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
> mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
> inc_zone_page_state(page, NR_WRITEBACK);
> }
> - mem_cgroup_end_page_stat(memcg, &locked, &memcg_flags);
> + mem_cgroup_end_page_stat(memcg);
> return ret;
>
> }
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c325f8bd2cc4..5e995a9da902 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1112,24 +1112,20 @@ void page_add_new_anon_rmap(struct page *page,
> void page_add_file_rmap(struct page *page)
> {
> struct mem_cgroup *memcg;
> - unsigned long flags;
> - bool locked;
>
> - memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
> + memcg = mem_cgroup_begin_page_stat(page);
> if (atomic_inc_and_test(&page->_mapcount)) {
> __inc_zone_page_state(page, NR_FILE_MAPPED);
> mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);
> }
> - mem_cgroup_end_page_stat(memcg, &locked, &flags);
> + mem_cgroup_end_page_stat(memcg);
> }
>
> static void page_remove_file_rmap(struct page *page)
> {
> struct mem_cgroup *memcg;
> - unsigned long flags;
> - bool locked;
>
> - memcg = mem_cgroup_begin_page_stat(page, &locked, &flags);
> + memcg = mem_cgroup_begin_page_stat(page);
>
> /* page still mapped by someone else? */
> if (!atomic_add_negative(-1, &page->_mapcount))
> @@ -1150,7 +1146,7 @@ static void page_remove_file_rmap(struct page *page)
> if (unlikely(PageMlocked(page)))
> clear_page_mlock(page);
> out:
> - mem_cgroup_end_page_stat(memcg, &locked, &flags);
> + mem_cgroup_end_page_stat(memcg);
> }
>
> /**
> --
> 2.2.0
>
--
Michal Hocko
SUSE Labs
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