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-rw-r--r--mm/page-writeback.c83
1 files changed, 81 insertions, 2 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4b954c9fe846..1721b6523c04 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -777,6 +777,79 @@ static void global_update_bandwidth(unsigned long thresh,
spin_unlock(&dirty_lock);
}
+/*
+ * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.
+ *
+ * Normal bdi tasks will be curbed at or below it in long term.
+ * Obviously it should be around (write_bw / N) when there are N dd tasks.
+ */
+static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
+ unsigned long thresh,
+ unsigned long bg_thresh,
+ unsigned long dirty,
+ unsigned long bdi_thresh,
+ unsigned long bdi_dirty,
+ unsigned long dirtied,
+ unsigned long elapsed)
+{
+ 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;
+
+ /*
+ * The dirty rate will match the writeout rate in long term, except
+ * when dirty pages are truncated by userspace or re-dirtied by FS.
+ */
+ dirty_rate = (dirtied - bdi->dirtied_stamp) * HZ / elapsed;
+
+ pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty,
+ bdi_thresh, bdi_dirty);
+ /*
+ * task_ratelimit reflects each dd's dirty rate for the past 200ms.
+ */
+ task_ratelimit = (u64)dirty_ratelimit *
+ pos_ratio >> RATELIMIT_CALC_SHIFT;
+ task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
+
+ /*
+ * A linear estimation of the "balanced" throttle rate. The theory is,
+ * if there are N dd tasks, each throttled at task_ratelimit, the bdi's
+ * dirty_rate will be measured to be (N * task_ratelimit). So the below
+ * formula will yield the balanced rate limit (write_bw / N).
+ *
+ * Note that the expanded form is not a pure rate feedback:
+ * rate_(i+1) = rate_(i) * (write_bw / dirty_rate) (1)
+ * but also takes pos_ratio into account:
+ * rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio (2)
+ *
+ * (1) is not realistic because pos_ratio also takes part in balancing
+ * the dirty rate. Consider the state
+ * pos_ratio = 0.5 (3)
+ * rate = 2 * (write_bw / N) (4)
+ * If (1) is used, it will stuck in that state! Because each dd will
+ * be throttled at
+ * task_ratelimit = pos_ratio * rate = (write_bw / N) (5)
+ * yielding
+ * dirty_rate = N * task_ratelimit = write_bw (6)
+ * put (6) into (1) we get
+ * rate_(i+1) = rate_(i) (7)
+ *
+ * So we end up using (2) to always keep
+ * rate_(i+1) ~= (write_bw / N) (8)
+ * regardless of the value of pos_ratio. As long as (8) is satisfied,
+ * pos_ratio is able to drive itself to 1.0, which is not only where
+ * the dirty count meet the setpoint, but also where the slope of
+ * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
+ */
+ balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
+ dirty_rate | 1);
+
+ bdi->dirty_ratelimit = max(balanced_dirty_ratelimit, 1UL);
+}
+
void __bdi_update_bandwidth(struct backing_dev_info *bdi,
unsigned long thresh,
unsigned long bg_thresh,
@@ -787,6 +860,7 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
{
unsigned long now = jiffies;
unsigned long elapsed = now - bdi->bw_time_stamp;
+ unsigned long dirtied;
unsigned long written;
/*
@@ -795,6 +869,7 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
if (elapsed < BANDWIDTH_INTERVAL)
return;
+ dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]);
written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
/*
@@ -804,12 +879,16 @@ void __bdi_update_bandwidth(struct backing_dev_info *bdi,
if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
goto snapshot;
- if (thresh)
+ if (thresh) {
global_update_bandwidth(thresh, dirty, now);
-
+ bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty,
+ bdi_thresh, bdi_dirty,
+ dirtied, elapsed);
+ }
bdi_update_write_bandwidth(bdi, elapsed, written);
snapshot:
+ bdi->dirtied_stamp = dirtied;
bdi->written_stamp = written;
bdi->bw_time_stamp = now;
}