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-rw-r--r--mm/page-writeback.c71
1 files changed, 70 insertions, 1 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 1721b6523c04..d4a6e91bd9e5 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -792,12 +792,17 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
unsigned long dirtied,
unsigned long elapsed)
{
+ unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
+ unsigned long limit = hard_dirty_limit(thresh);
+ unsigned long setpoint = (freerun + limit) / 2;
unsigned long write_bw = bdi->avg_write_bandwidth;
unsigned long dirty_ratelimit = bdi->dirty_ratelimit;
unsigned long dirty_rate;
unsigned long task_ratelimit;
unsigned long balanced_dirty_ratelimit;
unsigned long pos_ratio;
+ unsigned long step;
+ unsigned long x;
/*
* The dirty rate will match the writeout rate in long term, except
@@ -847,7 +852,71 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
dirty_rate | 1);
- bdi->dirty_ratelimit = max(balanced_dirty_ratelimit, 1UL);
+ /*
+ * We could safely do this and return immediately:
+ *
+ * bdi->dirty_ratelimit = balanced_dirty_ratelimit;
+ *
+ * However to get a more stable dirty_ratelimit, the below elaborated
+ * code makes use of task_ratelimit to filter out sigular points and
+ * limit the step size.
+ *
+ * The below code essentially only uses the relative value of
+ *
+ * task_ratelimit - dirty_ratelimit
+ * = (pos_ratio - 1) * dirty_ratelimit
+ *
+ * which reflects the direction and size of dirty position error.
+ */
+
+ /*
+ * dirty_ratelimit will follow balanced_dirty_ratelimit iff
+ * task_ratelimit is on the same side of dirty_ratelimit, too.
+ * For example, when
+ * - dirty_ratelimit > balanced_dirty_ratelimit
+ * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
+ * lowering dirty_ratelimit will help meet both the position and rate
+ * control targets. Otherwise, don't update dirty_ratelimit if it will
+ * only help meet the rate target. After all, what the users ultimately
+ * feel and care are stable dirty rate and small position error.
+ *
+ * |task_ratelimit - dirty_ratelimit| is used to limit the step size
+ * and filter out the sigular points of balanced_dirty_ratelimit. Which
+ * keeps jumping around randomly and can even leap far away at times
+ * due to the small 200ms estimation period of dirty_rate (we want to
+ * keep that period small to reduce time lags).
+ */
+ step = 0;
+ if (dirty < setpoint) {
+ x = min(bdi->balanced_dirty_ratelimit,
+ min(balanced_dirty_ratelimit, task_ratelimit));
+ if (dirty_ratelimit < x)
+ step = x - dirty_ratelimit;
+ } else {
+ x = max(bdi->balanced_dirty_ratelimit,
+ max(balanced_dirty_ratelimit, task_ratelimit));
+ if (dirty_ratelimit > x)
+ step = dirty_ratelimit - x;
+ }
+
+ /*
+ * Don't pursue 100% rate matching. It's impossible since the balanced
+ * rate itself is constantly fluctuating. So decrease the track speed
+ * when it gets close to the target. Helps eliminate pointless tremors.
+ */
+ step >>= dirty_ratelimit / (2 * step + 1);
+ /*
+ * Limit the tracking speed to avoid overshooting.
+ */
+ step = (step + 7) / 8;
+
+ if (dirty_ratelimit < balanced_dirty_ratelimit)
+ dirty_ratelimit += step;
+ else
+ dirty_ratelimit -= step;
+
+ bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
+ bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
}
void __bdi_update_bandwidth(struct backing_dev_info *bdi,