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-rw-r--r--mm/page-writeback.c191
1 files changed, 190 insertions, 1 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0e6dd5c2..c16ddd8 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -46,6 +46,8 @@
*/
#define BANDWIDTH_INTERVAL max(HZ/5, 1)
+#define RATELIMIT_CALC_SHIFT 10
+
/*
* After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
* will look to see if it needs to force writeback or throttling.
@@ -411,6 +413,12 @@ unsigned long determine_dirtyable_memory(void)
return x + 1; /* Ensure that we never return 0 */
}
+static unsigned long dirty_freerun_ceiling(unsigned long thresh,
+ unsigned long bg_thresh)
+{
+ return (thresh + bg_thresh) / 2;
+}
+
static unsigned long hard_dirty_limit(unsigned long thresh)
{
return max(thresh, global_dirty_limit);
@@ -495,6 +503,184 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
return bdi_dirty;
}
+/*
+ * Dirty position control.
+ *
+ * (o) global/bdi setpoints
+ *
+ * We want the dirty pages be balanced around the global/bdi setpoints.
+ * When the number of dirty pages is higher/lower than the setpoint, the
+ * dirty position control ratio (and hence task dirty ratelimit) will be
+ * decreased/increased to bring the dirty pages back to the setpoint.
+ *
+ * pos_ratio = 1 << RATELIMIT_CALC_SHIFT
+ *
+ * if (dirty < setpoint) scale up pos_ratio
+ * if (dirty > setpoint) scale down pos_ratio
+ *
+ * if (bdi_dirty < bdi_setpoint) scale up pos_ratio
+ * if (bdi_dirty > bdi_setpoint) scale down pos_ratio
+ *
+ * task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
+ *
+ * (o) global control line
+ *
+ * ^ pos_ratio
+ * |
+ * | |<===== global dirty control scope ======>|
+ * 2.0 .............*
+ * | .*
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * 1.0 ................................*
+ * | . . *
+ * | . . *
+ * | . . *
+ * | . . *
+ * | . . *
+ * 0 +------------.------------------.----------------------*------------->
+ * freerun^ setpoint^ limit^ dirty pages
+ *
+ * (o) bdi control line
+ *
+ * ^ pos_ratio
+ * |
+ * | *
+ * | *
+ * | *
+ * | *
+ * | * |<=========== span ============>|
+ * 1.0 .......................*
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * | . *
+ * 1/4 ...............................................* * * * * * * * * * * *
+ * | . .
+ * | . .
+ * | . .
+ * 0 +----------------------.-------------------------------.------------->
+ * bdi_setpoint^ x_intercept^
+ *
+ * The bdi control line won't drop below pos_ratio=1/4, so that bdi_dirty can
+ * be smoothly throttled down to normal if it starts high in situations like
+ * - start writing to a slow SD card and a fast disk at the same time. The SD
+ * card's bdi_dirty may rush to many times higher than bdi_setpoint.
+ * - the bdi dirty thresh drops quickly due to change of JBOD workload
+ */
+static unsigned long bdi_position_ratio(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 write_bw = bdi->avg_write_bandwidth;
+ unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
+ unsigned long limit = hard_dirty_limit(thresh);
+ unsigned long x_intercept;
+ unsigned long setpoint; /* dirty pages' target balance point */
+ unsigned long bdi_setpoint;
+ unsigned long span;
+ long long pos_ratio; /* for scaling up/down the rate limit */
+ long x;
+
+ if (unlikely(dirty >= limit))
+ return 0;
+
+ /*
+ * global setpoint
+ *
+ * setpoint - dirty 3
+ * f(dirty) := 1.0 + (----------------)
+ * limit - setpoint
+ *
+ * it's a 3rd order polynomial that subjects to
+ *
+ * (1) f(freerun) = 2.0 => rampup dirty_ratelimit reasonably fast
+ * (2) f(setpoint) = 1.0 => the balance point
+ * (3) f(limit) = 0 => the hard limit
+ * (4) df/dx <= 0 => negative feedback control
+ * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
+ * => fast response on large errors; small oscillation near setpoint
+ */
+ setpoint = (freerun + limit) / 2;
+ x = div_s64((setpoint - dirty) << RATELIMIT_CALC_SHIFT,
+ limit - setpoint + 1);
+ pos_ratio = x;
+ pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
+ pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
+ pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
+
+ /*
+ * We have computed basic pos_ratio above based on global situation. If
+ * the bdi is over/under its share of dirty pages, we want to scale
+ * pos_ratio further down/up. That is done by the following mechanism.
+ */
+
+ /*
+ * bdi setpoint
+ *
+ * f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
+ *
+ * x_intercept - bdi_dirty
+ * := --------------------------
+ * x_intercept - bdi_setpoint
+ *
+ * The main bdi control line is a linear function that subjects to
+ *
+ * (1) f(bdi_setpoint) = 1.0
+ * (2) k = - 1 / (8 * write_bw) (in single bdi case)
+ * or equally: x_intercept = bdi_setpoint + 8 * write_bw
+ *
+ * For single bdi case, the dirty pages are observed to fluctuate
+ * regularly within range
+ * [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
+ * for various filesystems, where (2) can yield in a reasonable 12.5%
+ * fluctuation range for pos_ratio.
+ *
+ * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate up to its
+ * own size, so move the slope over accordingly and choose a slope that
+ * yields 100% pos_ratio fluctuation on suddenly doubled bdi_thresh.
+ */
+ if (unlikely(bdi_thresh > thresh))
+ bdi_thresh = thresh;
+ /*
+ * scale global setpoint to bdi's:
+ * bdi_setpoint = setpoint * bdi_thresh / thresh
+ */
+ x = div_u64((u64)bdi_thresh << 16, thresh + 1);
+ bdi_setpoint = setpoint * (u64)x >> 16;
+ /*
+ * Use span=(8*write_bw) in single bdi case as indicated by
+ * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD case.
+ *
+ * bdi_thresh thresh - bdi_thresh
+ * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh
+ * thresh thresh
+ */
+ span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
+ x_intercept = bdi_setpoint + span;
+
+ if (bdi_dirty < x_intercept - span / 4) {
+ pos_ratio *= x_intercept - bdi_dirty;
+ do_div(pos_ratio, x_intercept - bdi_setpoint + 1);
+ } else
+ pos_ratio /= 4;
+
+ return pos_ratio;
+}
+
static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
unsigned long elapsed,
unsigned long written)
@@ -655,6 +841,7 @@ static void balance_dirty_pages(struct address_space *mapping,
unsigned long nr_reclaimable, bdi_nr_reclaimable;
unsigned long nr_dirty; /* = file_dirty + writeback + unstable_nfs */
unsigned long bdi_dirty;
+ unsigned long freerun;
unsigned long background_thresh;
unsigned long dirty_thresh;
unsigned long bdi_thresh;
@@ -679,7 +866,9 @@ static void balance_dirty_pages(struct address_space *mapping,
* catch-up. This avoids (excessively) small writeouts
* when the bdi limits are ramping up.
*/
- if (nr_dirty <= (background_thresh + dirty_thresh) / 2)
+ freerun = dirty_freerun_ceiling(dirty_thresh,
+ background_thresh);
+ if (nr_dirty <= freerun)
break;
bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
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