[PATCH v9 11/13] writeback: make background writeback cgroup aware

From: Greg Thelen
Date: Wed Aug 17 2011 - 12:19:51 EST

When the system is under background dirty memory threshold but some
cgroups are over their background dirty memory thresholds, then only
writeback inodes associated with the over-limit cgroups.

In addition to checking if the system dirty memory usage is over the
system background threshold, over_bground_thresh() now checks if any
cgroups are over their respective background dirty memory thresholds.

If over-limit cgroups are found, then the new
wb_writeback_work.for_cgroup field is set to distinguish between system
and memcg overages. The new wb_writeback_work.shared_inodes field is
also set. Inodes written by multiple cgroup are marked owned by
I_MEMCG_SHARED rather than a particular cgroup. Such shared inodes
cannot easily be attributed to a cgroup, so per-cgroup writeback
(futures version of wakeup_flusher_threads and balance_dirty_pages)
performs suboptimally in the presence of shared inodes. Therefore,
write shared inodes when performing cgroup background writeback.

If performing cgroup writeback, move_expired_inodes() skips inodes that
do not contribute dirty pages to the cgroup being written back.

After writing some pages, wb_writeback() will call
mem_cgroup_writeback_done() to update the set of over-bg-limits memcg.

This change also makes wakeup_flusher_threads() memcg aware so that
per-cgroup try_to_free_pages() is able to operate more efficiently
without having to write pages of foreign containers. This change adds a
mem_cgroup parameter to wakeup_flusher_threads() to allow callers,
especially try_to_free_pages() and foreground writeback from
balance_dirty_pages(), to specify a particular cgroup to write inodes
from.

Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
---
Changelog since v8:

- Added optional memcg parameter to __bdi_start_writeback(),
bdi_start_writeback(), wakeup_flusher_threads(), writeback_inodes_wb().

- move_expired_inodes() now uses pass in struct wb_writeback_work instead of
struct writeback_control.

- Added comments to over_bground_thresh().

fs/buffer.c | 2 +-
fs/fs-writeback.c | 96 +++++++++++++++++++++++++++++++++-----------
fs/sync.c | 2 +-
include/linux/writeback.h | 6 ++-
mm/backing-dev.c | 3 +-
mm/page-writeback.c | 3 +-
mm/vmscan.c | 3 +-
7 files changed, 84 insertions(+), 31 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index dd0220b..da1fb23 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -293,7 +293,7 @@ static void free_more_memory(void)
struct zone *zone;
int nid;

- wakeup_flusher_threads(1024);
+ wakeup_flusher_threads(1024, NULL);
yield();

for_each_online_node(nid) {
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e91fb82..ba55336 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -38,10 +38,14 @@ struct wb_writeback_work {
struct super_block *sb;
unsigned long *older_than_this;
enum writeback_sync_modes sync_mode;
+ unsigned short memcg_id; /* If non-zero, then writeback specified
+ * cgroup. */
unsigned int tagged_writepages:1;
unsigned int for_kupdate:1;
unsigned int range_cyclic:1;
unsigned int for_background:1;
+ unsigned int for_cgroup:1; /* cgroup writeback */
+ unsigned int shared_inodes:1; /* write inodes spanning cgroups */

struct list_head list; /* pending work list */
struct completion *done; /* set if the caller waits */
@@ -114,9 +118,12 @@ static void bdi_queue_work(struct backing_dev_info *bdi,
spin_unlock_bh(&bdi->wb_lock);
}

+/*
+ * @memcg is optional. If set, then limit writeback to the specified cgroup.
+ */
static void
__bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
- bool range_cyclic)
+ bool range_cyclic, struct mem_cgroup *memcg)
{
struct wb_writeback_work *work;

@@ -136,6 +143,8 @@ __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
work->sync_mode = WB_SYNC_NONE;
work->nr_pages = nr_pages;
work->range_cyclic = range_cyclic;
+ work->memcg_id = memcg ? css_id(mem_cgroup_css(memcg)) : 0;
+ work->for_cgroup = memcg != NULL;

bdi_queue_work(bdi, work);
}
@@ -153,7 +162,7 @@ __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
*/
void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages)
{
- __bdi_start_writeback(bdi, nr_pages, true);
+ __bdi_start_writeback(bdi, nr_pages, true, NULL);
}

/**
@@ -257,15 +266,20 @@ static int move_expired_inodes(struct list_head *delaying_queue,
LIST_HEAD(tmp);
struct list_head *pos, *node;
struct super_block *sb = NULL;
- struct inode *inode;
+ struct inode *inode, *tmp_inode;
int do_sb_sort = 0;
int moved = 0;

- while (!list_empty(delaying_queue)) {
- inode = wb_inode(delaying_queue->prev);
+ list_for_each_entry_safe_reverse(inode, tmp_inode, delaying_queue,
+ i_wb_list) {
if (work->older_than_this &&
inode_dirtied_after(inode, *work->older_than_this))
break;
+ if (work->for_cgroup &&
+ !should_writeback_mem_cgroup_inode(inode,
+ work->memcg_id,
+ work->shared_inodes))
+ continue;
if (sb && sb != inode->i_sb)
do_sb_sort = 1;
sb = inode->i_sb;
@@ -643,31 +657,63 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
return wrote;
}

-long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages)
+/*
+ * @memcg is optional. If set, then limit writeback to the specified cgroup.
+ * If @shared_inodes is set then writeback inodes shared by several memcg.
+ */
+long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,
+ struct mem_cgroup *memcg, bool shared_inodes)
{
struct wb_writeback_work work = {
.nr_pages = nr_pages,
.sync_mode = WB_SYNC_NONE,
+ .memcg_id = memcg ? css_id(mem_cgroup_css(memcg)) : 0,
+ .for_cgroup = (memcg != NULL) || shared_inodes,
+ .shared_inodes = shared_inodes,
.range_cyclic = 1,
};

spin_lock(&wb->list_lock);
if (list_empty(&wb->b_io))
- queue_io(wb, NULL);
+ queue_io(wb, &work);
__writeback_inodes_wb(wb, &work);
spin_unlock(&wb->list_lock);

return nr_pages - work.nr_pages;
}

-static inline bool over_bground_thresh(void)
+static inline bool over_bground_thresh(struct wb_writeback_work *work)
{
unsigned long background_thresh, dirty_thresh;

global_dirty_limits(&background_thresh, &dirty_thresh);

- return (global_page_state(NR_FILE_DIRTY) +
- global_page_state(NR_UNSTABLE_NFS) > background_thresh);
+ if (global_page_state(NR_FILE_DIRTY) +
+ global_page_state(NR_UNSTABLE_NFS) > background_thresh) {
+ work->for_cgroup = 0;
+ return true;
+ }
+
+ /*
+ * System dirty memory is below system background limit. Check if any
+ * memcg are over memcg background limit.
+ */
+ if (mem_cgroups_over_bground_dirty_thresh()) {
+ work->for_cgroup = 1;
+
+ /*
+ * Set shared_inodes so that background flusher writes shared
+ * inodes in addition to inodes in over-limit memcg. Such
+ * shared inodes should be rarer than inodes written by a single
+ * memcg. Shared inodes limit the ability to map from memcg to
+ * inode in wakeup_flusher_threads() and writeback_inodes_wb().
+ * So the quicker such shared inodes are written, the better.
+ */
+ work->shared_inodes = 1;
+ return true;
+ }
+
+ return false;
}

/*
@@ -729,7 +775,7 @@ static long wb_writeback(struct bdi_writeback *wb,
* For background writeout, stop when we are below the
* background dirty threshold
*/
- if (work->for_background && !over_bground_thresh())
+ if (work->for_background && !over_bground_thresh(work))
break;

if (work->for_kupdate) {
@@ -749,6 +795,9 @@ static long wb_writeback(struct bdi_writeback *wb,

wb_update_bandwidth(wb, wb_start);

+ if (progress)
+ mem_cgroup_writeback_done();
+
/*
* Did we write something? Try for more
*
@@ -813,17 +862,15 @@ static unsigned long get_nr_dirty_pages(void)

static long wb_check_background_flush(struct bdi_writeback *wb)
{
- if (over_bground_thresh()) {
-
- struct wb_writeback_work work = {
- .nr_pages = LONG_MAX,
- .sync_mode = WB_SYNC_NONE,
- .for_background = 1,
- .range_cyclic = 1,
- };
+ struct wb_writeback_work work = {
+ .nr_pages = LONG_MAX,
+ .sync_mode = WB_SYNC_NONE,
+ .for_background = 1,
+ .range_cyclic = 1,
+ };

+ if (over_bground_thresh(&work))
return wb_writeback(wb, &work);
- }

return 0;
}
@@ -968,10 +1015,11 @@ int bdi_writeback_thread(void *data)


/*
- * Start writeback of `nr_pages' pages. If `nr_pages' is zero, write back
- * the whole world.
+ * Start writeback of `nr_pages' pages. If `nr_pages' is zero, write back the
+ * whole world. If 'memcg' is non-NULL, then limit attempt to only write pages
+ * from the specified cgroup.
*/
-void wakeup_flusher_threads(long nr_pages)
+void wakeup_flusher_threads(long nr_pages, struct mem_cgroup *memcg)
{
struct backing_dev_info *bdi;

@@ -984,7 +1032,7 @@ void wakeup_flusher_threads(long nr_pages)
list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
if (!bdi_has_dirty_io(bdi))
continue;
- __bdi_start_writeback(bdi, nr_pages, false);
+ __bdi_start_writeback(bdi, nr_pages, false, memcg);
}
rcu_read_unlock();
}
diff --git a/fs/sync.c b/fs/sync.c
index c98a747..7c1ba55 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -98,7 +98,7 @@ static void sync_filesystems(int wait)
*/
SYSCALL_DEFINE0(sync)
{
- wakeup_flusher_threads(0);
+ wakeup_flusher_threads(0, NULL);
sync_filesystems(0);
sync_filesystems(1);
if (unlikely(laptop_mode))
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index d12d070..e6790e8 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -40,6 +40,7 @@
#define MIN_WRITEBACK_PAGES (4096UL >> (PAGE_CACHE_SHIFT - 10))

struct backing_dev_info;
+struct mem_cgroup;

/*
* fs/fs-writeback.c
@@ -85,9 +86,10 @@ void writeback_inodes_sb_nr(struct super_block *, unsigned long nr);
int writeback_inodes_sb_if_idle(struct super_block *);
int writeback_inodes_sb_nr_if_idle(struct super_block *, unsigned long nr);
void sync_inodes_sb(struct super_block *);
-long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages);
+long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,
+ struct mem_cgroup *memcg, bool shared_inodes);
long wb_do_writeback(struct bdi_writeback *wb, int force_wait);
-void wakeup_flusher_threads(long nr_pages);
+void wakeup_flusher_threads(long nr_pages, struct mem_cgroup *memcg);

/* writeback.h requires fs.h; it, too, is not included from here. */
static inline void wait_on_inode(struct inode *inode)
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index d6edf8d..60d101d 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -456,7 +456,8 @@ static int bdi_forker_thread(void *ptr)
* the bdi from the thread. Hopefully 1024 is
* large enough for efficient IO.
*/
- writeback_inodes_wb(&bdi->wb, 1024);
+ writeback_inodes_wb(&bdi->wb, 1024, NULL,
+ false);
} else {
/*
* The spinlock makes sure we do not lose
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 12b3900..64de98c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -736,7 +736,8 @@ static void balance_dirty_pages(struct address_space *mapping,
trace_balance_dirty_start(bdi);
if (bdi_nr_reclaimable > task_bdi_thresh) {
pages_written += writeback_inodes_wb(&bdi->wb,
- write_chunk);
+ write_chunk,
+ NULL, false);
trace_balance_dirty_written(bdi, pages_written);
if (pages_written >= write_chunk)
break; /* We've done our duty */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3153729..fb0ae99 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2223,7 +2223,8 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
*/
writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
if (total_scanned > writeback_threshold) {
- wakeup_flusher_threads(laptop_mode ? 0 : total_scanned);
+ wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
+ sc->mem_cgroup);
sc->may_writepage = 1;
}

--
1.7.3.1

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