[PATCH 09/19] writeback: add dirty_throttle_control->pos_ratio

Tejun Heo
2015-04-06T20:05:08Z

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>
---
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