writeback: add dirty_throttle_control->pos_ratio

wb_position_ratio() is used to calculate pos_ratio, which is used for
two purposes.  wb_update_dirty_ratelimit() uses it to adjust
wb->[balanced_]dirty_ratelimit gradually and balance_dirty_pages() to
immediately adjust dirty_ratelimit right before applying it to
determine pause duration.

While wb_update_dirty_ratelimit() is separately rate limited from
balance_dirty_pages(), on the run where the ratelimit is updated, we
end up calculating pos_ratio twice with the same parameters.

This patch adds dirty_throttle_control->pos_ratio.
balance_dirty_pages() calculates it once per run and
wb_update_dirty_ratelimit() uses the value stored in
dirty_throttle_control.

This removes the duplicate calculation and also will help implementing
memcg wb_domain.

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, 21 insertions, 15 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2352c69bff12..fcebae7db874 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -135,6 +135,8 @@ struct dirty_throttle_control {
         unsigned long           wb_dirty;       /* per-wb counterparts */
         unsigned long           wb_thresh;
         unsigned long           wb_bg_thresh;
+
+        unsigned long           pos_ratio;
 };
 
 #define GDTC_INIT(__wb)         .wb = (__wb)
@@ -717,7 +719,7 @@ static long long pos_ratio_polynom(unsigned long setpoint,
  *   card's wb_dirty may rush to many times higher than wb_setpoint.
  * - the wb dirty thresh drops quickly due to change of JBOD workload
  */
-static unsigned long wb_position_ratio(struct dirty_throttle_control *dtc)
+static void wb_position_ratio(struct dirty_throttle_control *dtc)
 {
         struct bdi_writeback *wb = dtc->wb;
         unsigned long write_bw = wb->avg_write_bandwidth;
@@ -731,8 +733,10 @@ static unsigned long wb_position_ratio(struct dirty_throttle_control *dtc)
         long long pos_ratio;            /* for scaling up/down the rate limit */
         long x;
 
+        dtc->pos_ratio = 0;
+
         if (unlikely(dtc->dirty >= limit))
-                return 0;
+                return;
 
         /*
          * global setpoint
@@ -770,18 +774,20 @@ static unsigned long wb_position_ratio(struct dirty_throttle_control *dtc)
         if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
                 long long wb_pos_ratio;
 
-                if (dtc->wb_dirty < 8)
-                        return min_t(long long, pos_ratio * 2,
-                                     2 << RATELIMIT_CALC_SHIFT);
+                if (dtc->wb_dirty < 8) {
+                        dtc->pos_ratio = min_t(long long, pos_ratio * 2,
+                                           2 << RATELIMIT_CALC_SHIFT);
+                        return;
+                }
 
                 if (dtc->wb_dirty >= wb_thresh)
-                        return 0;
+                        return;
 
                 wb_setpoint = dirty_freerun_ceiling(wb_thresh,
                                                     dtc->wb_bg_thresh);
 
                 if (wb_setpoint == 0 || wb_setpoint == wb_thresh)
-                        return 0;
+                        return;
 
                 wb_pos_ratio = pos_ratio_polynom(wb_setpoint, dtc->wb_dirty,
                                                  wb_thresh);
@@ -807,7 +813,8 @@ static unsigned long wb_position_ratio(struct dirty_throttle_control *dtc)
                  * is 2. We might want to tweak this if we observe the control
                  * system is too slow to adapt.
                  */
-                return min(pos_ratio, wb_pos_ratio);
+                dtc->pos_ratio = min(pos_ratio, wb_pos_ratio);
+                return;
         }
 
         /*
@@ -888,7 +895,7 @@ static unsigned long wb_position_ratio(struct dirty_throttle_control *dtc)
                         pos_ratio *= 8;
         }
 
-        return pos_ratio;
+        dtc->pos_ratio = pos_ratio;
 }
 
 static void wb_update_write_bandwidth(struct bdi_writeback *wb,
@@ -1009,7 +1016,6 @@ static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
         unsigned long dirty_rate;
         unsigned long task_ratelimit;
         unsigned long balanced_dirty_ratelimit;
-        unsigned long pos_ratio;
         unsigned long step;
         unsigned long x;
 
@@ -1019,12 +1025,11 @@ static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
          */
         dirty_rate = (dirtied - wb->dirtied_stamp) * HZ / elapsed;
 
-        pos_ratio = wb_position_ratio(dtc);
         /*
          * task_ratelimit reflects each dd's dirty rate for the past 200ms.
          */
         task_ratelimit = (u64)dirty_ratelimit *
-                                        pos_ratio >> RATELIMIT_CALC_SHIFT;
+                                        dtc->pos_ratio >> RATELIMIT_CALC_SHIFT;
         task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
 
         /*
@@ -1375,7 +1380,6 @@ static void balance_dirty_pages(struct address_space *mapping,
         bool dirty_exceeded = false;
         unsigned long task_ratelimit;
         unsigned long dirty_ratelimit;
-        unsigned long pos_ratio;
         struct backing_dev_info *bdi = wb->bdi;
         bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
         unsigned long start_time = jiffies;
@@ -1433,6 +1437,9 @@ static void balance_dirty_pages(struct address_space *mapping,
 
                 dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
                         ((gdtc->dirty > gdtc->thresh) || strictlimit);
+
+                wb_position_ratio(gdtc);
+
                 if (dirty_exceeded && !wb->dirty_exceeded)
                         wb->dirty_exceeded = 1;
 
@@ -1444,8 +1451,7 @@ static void balance_dirty_pages(struct address_space *mapping,
                 }
 
                 dirty_ratelimit = wb->dirty_ratelimit;
-                pos_ratio = wb_position_ratio(gdtc);
-                task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
+                task_ratelimit = ((u64)dirty_ratelimit * gdtc->pos_ratio) >>
                                                         RATELIMIT_CALC_SHIFT;
                 max_pause = wb_max_pause(wb, gdtc->wb_dirty);
                 min_pause = wb_min_pause(wb, max_pause,