[patch 1/3] mm: memcontrol: take a css reference for each charged page

Johannes Weiner
2014-09-20T20:00:33Z

Charges currently pin the css indirectly by playing tricks during
css_offline(): user pages stall the offlining process until all of
them have been reparented, whereas kmemcg acquires a keep-alive
reference if outstanding kernel pages are detected at that point.

In preparation for removing all this complexity, make the pinning
explicit and acquire a css references for every charged page.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
include/linux/cgroup.h | 26 +++++++++++++++++++++++++
include/linux/percpu-refcount.h | 43 ++++++++++++++++++++++++++++++++---------
mm/memcontrol.c | 17 +++++++++++++++-
3 files changed, 76 insertions(+), 10 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b5223c570eba..a9fe70d9c7c5 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -113,6 +113,19 @@ static inline void css_get(struct cgroup_subsys_state *css)
}

/**
+ * css_get_many - obtain references on the specified css
+ * @css: target css
+ * @n: number of references to get
+ *
+ * The caller must already have a reference.
+ */
+static inline void css_get_many(struct cgroup_subsys_state *css, unsigned int n)
+{
+ if (!(css->flags & CSS_NO_REF))
+ percpu_ref_get_many(&css->refcnt, n);
+}
+
+/**
* css_tryget - try to obtain a reference on the specified css
* @css: target css
*
@@ -159,6 +172,19 @@ static inline void css_put(struct cgroup_subsys_state *css)
percpu_ref_put(&css->refcnt);
}

+/**
+ * css_put_many - put css references
+ * @css: target css
+ * @n: number of references to put
+ *
+ * Put references obtained via css_get() and css_tryget_online().
+ */
+static inline void css_put_many(struct cgroup_subsys_state *css, unsigned int n)
+{
+ if (!(css->flags & CSS_NO_REF))
+ percpu_ref_put_many(&css->refcnt, n);
+}
+
/* bits in struct cgroup flags field */
enum {
/*
diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index b1973ba1d5f3..a4551456e06f 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -112,26 +112,38 @@ static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
}

/**
- * percpu_ref_get - increment a percpu refcount
+ * percpu_ref_get_many - increment a percpu refcount
* @ref: percpu_ref to get
+ * @nr: number of references to get
*
- * Analagous to atomic_inc().
+ * Analagous to atomic_add().
*/
-static inline void percpu_ref_get(struct percpu_ref *ref)
+static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
{
unsigned long __percpu *pcpu_count;

rcu_read_lock_sched();

if (__pcpu_ref_alive(ref, &pcpu_count))
- this_cpu_inc(*pcpu_count);
+ this_cpu_add(*pcpu_count, nr);
else
- atomic_long_inc(&ref->count);
+ atomic_long_add(nr, &ref->count);

rcu_read_unlock_sched();
}

/**
+ * percpu_ref_get - increment a percpu refcount
+ * @ref: percpu_ref to get
+ *
+ * Analagous to atomic_inc().
+ */
+static inline void percpu_ref_get(struct percpu_ref *ref)
+{
+ percpu_ref_get_many(ref, 1);
+}
+
+/**
* percpu_ref_tryget - try to increment a percpu refcount
* @ref: percpu_ref to try-get
*
@@ -191,27 +203,40 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
}

/**
- * percpu_ref_put - decrement a percpu refcount
+ * percpu_ref_put_many - decrement a percpu refcount
* @ref: percpu_ref to put
+ * @nr: number of references to put
*
* Decrement the refcount, and if 0, call the release function (which was passed
* to percpu_ref_init())
*/
-static inline void percpu_ref_put(struct percpu_ref *ref)
+static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
{
unsigned long __percpu *pcpu_count;

rcu_read_lock_sched();

if (__pcpu_ref_alive(ref, &pcpu_count))
- this_cpu_dec(*pcpu_count);
- else if (unlikely(atomic_long_dec_and_test(&ref->count)))
+ this_cpu_sub(*pcpu_count, nr);
+ else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
ref->release(ref);

rcu_read_unlock_sched();
}

/**
+ * percpu_ref_put - decrement a percpu refcount
+ * @ref: percpu_ref to put
+ *
+ * Decrement the refcount, and if 0, call the release function (which was passed
+ * to percpu_ref_init())
+ */
+static inline void percpu_ref_put(struct percpu_ref *ref)
+{
+ percpu_ref_put_many(ref, 1);
+}
+
+/**
* percpu_ref_is_zero - test whether a percpu refcount reached zero
* @ref: percpu_ref to test
*
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 154161bb7d4c..b832c87ec43b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2317,6 +2317,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
page_counter_uncharge(&old->memory, stock->nr_pages);
if (do_swap_account)
page_counter_uncharge(&old->memsw, stock->nr_pages);
+ css_put_many(&old->css, stock->nr_pages);
stock->nr_pages = 0;
}
stock->cached = NULL;
@@ -2573,6 +2574,7 @@ bypass:
return -EINTR;

done_restock:
+ css_get_many(&memcg->css, batch);
if (batch > nr_pages)
refill_stock(memcg, batch - nr_pages);
done:
@@ -2587,6 +2589,8 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
page_counter_uncharge(&memcg->memory, nr_pages);
if (do_swap_account)
page_counter_uncharge(&memcg->memsw, nr_pages);
+
+ css_put_many(&memcg->css, nr_pages);
}

/*
@@ -2788,6 +2792,7 @@ static int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp,
page_counter_charge(&memcg->memory, nr_pages, NULL);
if (do_swap_account)
page_counter_charge(&memcg->memsw, nr_pages, NULL);
+ css_get_many(&memcg->css, nr_pages);
ret = 0;
} else if (ret)
page_counter_uncharge(&memcg->kmem, nr_pages);
@@ -2803,8 +2808,10 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg,
page_counter_uncharge(&memcg->memsw, nr_pages);

/* Not down to 0 */
- if (page_counter_uncharge(&memcg->kmem, nr_pages))
+ if (page_counter_uncharge(&memcg->kmem, nr_pages)) {
+ css_put_many(&memcg->css, nr_pages);
return;
+ }

/*
* Releases a reference taken in kmem_cgroup_css_offline in case
@@ -2816,6 +2823,8 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg,
*/
if (memcg_kmem_test_and_clear_dead(memcg))
css_put(&memcg->css);
+
+ css_put_many(&memcg->css, nr_pages);
}

/*
@@ -3444,10 +3453,13 @@ static int mem_cgroup_move_parent(struct page *page,
ret = mem_cgroup_move_account(page, nr_pages,
pc, child, parent);
if (!ret) {
+ if (!mem_cgroup_is_root(parent))
+ css_get_many(&parent->css, nr_pages);
/* Take charge off the local counters */
page_counter_cancel(&child->memory, nr_pages);
if (do_swap_account)
page_counter_cancel(&child->memsw, nr_pages);
+ css_put_many(&child->css, nr_pages);
}

if (nr_pages > 1)
@@ -6379,6 +6391,9 @@ static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
__this_cpu_add(memcg->stat->nr_page_events, nr_anon + nr_file);
memcg_check_events(memcg, dummy_page);
local_irq_restore(flags);
+
+ if (!mem_cgroup_is_root(memcg))
+ css_put_many(&memcg->css, max(nr_mem, nr_memsw));
}

static void uncharge_list(struct list_head *page_list)
--
2.1.0