writeback: implement wb_domain

Dirtyable memory is distributed to a wb (bdi_writeback) according to
the relative bandwidth the wb is writing out in the whole system.
This distribution is global - each wb is measured against all other
wb's and gets the proportinately sized portion of the memory in the
whole system.

For cgroup writeback, the amount of dirtyable memory is scoped by
memcg and thus each wb would need to be measured and controlled in its
memcg.  IOW, a wb will belong to two writeback domains - the global
and memcg domains.

Currently, what constitutes the global writeback domain are scattered
across a number of global states.  This patch starts collecting them
into struct wb_domain.

* fprop_global which serves as the basis for proportional bandwidth
  measurement and its period timer are moved into struct wb_domain.

* global_wb_domain hosts the states for the global domain.

* While at it, flatten wb_writeout_fraction() into its callers.  This
  thin wrapper doesn't provide any actual benefits while getting in
  the way.

This is pure reorganization and doesn't introduce any behavioral
changes.

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>

1 files changed, 32 insertions, 0 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 80adf3d88d9d..3148db1296a2 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -7,6 +7,7 @@
 #include <linux/sched.h>
 #include <linux/workqueue.h>
 #include <linux/fs.h>
+#include <linux/flex_proportions.h>
 
 DECLARE_PER_CPU(int, dirty_throttle_leaks);
 
@@ -87,6 +88,36 @@ struct writeback_control {
 };
 
 /*
+ * A wb_domain represents a domain that wb's (bdi_writeback's) belong to
+ * and are measured against each other in.  There always is one global
+ * domain, global_wb_domain, that every wb in the system is a member of.
+ * This allows measuring the relative bandwidth of each wb to distribute
+ * dirtyable memory accordingly.
+ */
+struct wb_domain {
+        /*
+         * Scale the writeback cache size proportional to the relative
+         * writeout speed.
+         *
+         * We do this by keeping a floating proportion between BDIs, based
+         * on page writeback completions [end_page_writeback()]. Those
+         * devices that write out pages fastest will get the larger share,
+         * while the slower will get a smaller share.
+         *
+         * We use page writeout completions because we are interested in
+         * getting rid of dirty pages. Having them written out is the
+         * primary goal.
+         *
+         * We introduce a concept of time, a period over which we measure
+         * these events, because demand can/will vary over time. The length
+         * of this period itself is measured in page writeback completions.
+         */
+        struct fprop_global completions;
+        struct timer_list period_timer; /* timer for aging of completions */
+        unsigned long period_time;
+};
+
+/*
  * fs/fs-writeback.c
  */     
 struct bdi_writeback;
@@ -120,6 +151,7 @@ static inline void laptop_sync_completion(void) { }
 #endif
 void throttle_vm_writeout(gfp_t gfp_mask);
 bool zone_dirty_ok(struct zone *zone);
+int wb_domain_init(struct wb_domain *dom, gfp_t gfp);
 
 extern unsigned long global_dirty_limit;