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authorTejun Heo <tj@kernel.org>2015-05-28 14:50:51 -0400
committerJens Axboe <axboe@fb.com>2015-06-02 08:40:20 -0600
commit2a81490811d0296d390c571bb64eaa93e5ed7def (patch)
treea2716ba515488d22b4f2e428134c44f1f1674f5b /include/linux/writeback.h
parentb16b1deb553adcd7b3b7ce3e6d6fd1b923f314da (diff)
writeback: implement foreign cgroup inode detection
As concurrent write sharing of an inode is expected to be very rare and memcg only tracks page ownership on first-use basis severely confining the usefulness of such sharing, cgroup writeback tracks ownership per-inode. While the support for concurrent write sharing of an inode is deemed unnecessary, an inode being written to by different cgroups at different points in time is a lot more common, and, more importantly, charging only by first-use can too readily lead to grossly incorrect behaviors (single foreign page can lead to gigabytes of writeback to be incorrectly attributed). To resolve this issue, cgroup writeback detects the majority dirtier of an inode and will transfer the ownership to it. To avoid unnnecessary oscillation, the detection mechanism keeps track of history and gives out the switch verdict only if the foreign usage pattern is stable over a certain amount of time and/or writeback attempts. The detection mechanism has fairly low space and computation overhead. It adds 8 bytes to struct inode (one int and two u16's) and minimal amount of calculation per IO. The detection mechanism converges to the correct answer usually in several seconds of IO time when there's a clear majority dirtier. Even when there isn't, it can reach an acceptable answer fairly quickly under most circumstances. Please see wb_detach_inode() for more details. This patch only implements detection. Following patches will implement actual switching. v2: wbc_account_io() now checks whether the wbc is associated with a wb before dereferencing it. This can happen when pageout() is writing pages directly without going through the usual writeback path. As pageout() path is single-threaded, we don't want it to be blocked behind a slow cgroup and ultimately want it to delegate actual writing to the usual writeback path. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: Jan Kara <jack@suse.cz> Cc: Wu Fengguang <fengguang.wu@intel.com> Cc: Greg Thelen <gthelen@google.com> Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'include/linux/writeback.h')
-rw-r--r--include/linux/writeback.h16
1 files changed, 16 insertions, 0 deletions
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 8f964e558af5..b333c945e571 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -88,6 +88,15 @@ struct writeback_control {
unsigned for_sync:1; /* sync(2) WB_SYNC_ALL writeback */
#ifdef CONFIG_CGROUP_WRITEBACK
struct bdi_writeback *wb; /* wb this writeback is issued under */
+ struct inode *inode; /* inode being written out */
+
+ /* foreign inode detection, see wbc_detach_inode() */
+ int wb_id; /* current wb id */
+ int wb_lcand_id; /* last foreign candidate wb id */
+ int wb_tcand_id; /* this foreign candidate wb id */
+ size_t wb_bytes; /* bytes written by current wb */
+ size_t wb_lcand_bytes; /* bytes written by last candidate */
+ size_t wb_tcand_bytes; /* bytes written by this candidate */
#endif
};
@@ -187,6 +196,8 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
struct inode *inode)
__releases(&inode->i_lock);
void wbc_detach_inode(struct writeback_control *wbc);
+void wbc_account_io(struct writeback_control *wbc, struct page *page,
+ size_t bytes);
/**
* inode_attach_wb - associate an inode with its wb
@@ -285,6 +296,11 @@ static inline void wbc_init_bio(struct writeback_control *wbc, struct bio *bio)
{
}
+static inline void wbc_account_io(struct writeback_control *wbc,
+ struct page *page, size_t bytes)
+{
+}
+
#endif /* CONFIG_CGROUP_WRITEBACK */
/*