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 <t...@kernel.org>
Cc: Jens Axboe <ax...@kernel.dk>
Cc: Jan Kara <j...@suse.cz>
Cc: Wu Fengguang <fengguang...@intel.com>
Cc: Greg Thelen <gthe...@google.com>
---
 mm/page-writeback.c | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 33b1536..3450520 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,
-- 
2.1.0

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