bcache: New writeback PD controller

The old writeback PD controller could get into states where it had throttled all
the way down and take way too long to recover - it was too complicated to really
understand what it was doing.

This rewrites a good chunk of it to hopefully be simpler and make more sense,
and it also pays more attention to units which should make the behaviour a bit
easier to understand.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/bcache.h

@@ -373,14 +373,14 @@ struct cached_dev {
 	unsigned char		writeback_percent;
 	unsigned		writeback_delay;
 
-	int			writeback_rate_change;
-	int64_t			writeback_rate_derivative;
 	uint64_t		writeback_rate_target;
+	int64_t			writeback_rate_proportional;
+	int64_t			writeback_rate_derivative;
+	int64_t			writeback_rate_change;
 
 	unsigned		writeback_rate_update_seconds;
 	unsigned		writeback_rate_d_term;
 	unsigned		writeback_rate_p_term_inverse;
-	unsigned		writeback_rate_d_smooth;
 };
 
 enum alloc_watermarks {

drivers/md/bcache/sysfs.c

@@ -83,7 +83,6 @@ rw_attribute(writeback_rate);
 rw_attribute(writeback_rate_update_seconds);
 rw_attribute(writeback_rate_d_term);
 rw_attribute(writeback_rate_p_term_inverse);
-rw_attribute(writeback_rate_d_smooth);
 read_attribute(writeback_rate_debug);
 
 read_attribute(stripe_size);
@@ -129,31 +128,41 @@ SHOW(__bch_cached_dev)
 	var_printf(writeback_running,	"%i");
 	var_print(writeback_delay);
 	var_print(writeback_percent);
-	sysfs_print(writeback_rate,	dc->writeback_rate.rate);
+	sysfs_hprint(writeback_rate,	dc->writeback_rate.rate << 9);
 
 	var_print(writeback_rate_update_seconds);
 	var_print(writeback_rate_d_term);
 	var_print(writeback_rate_p_term_inverse);
-	var_print(writeback_rate_d_smooth);
 
 	if (attr == &sysfs_writeback_rate_debug) {
+		char rate[20];
 		char dirty[20];
-		char derivative[20];
 		char target[20];
-		bch_hprint(dirty,
-			   bcache_dev_sectors_dirty(&dc->disk) << 9);
-		bch_hprint(derivative,	dc->writeback_rate_derivative << 9);
+		char proportional[20];
+		char derivative[20];
+		char change[20];
+		s64 next_io;
+
+		bch_hprint(rate,	dc->writeback_rate.rate << 9);
+		bch_hprint(dirty,	bcache_dev_sectors_dirty(&dc->disk) << 9);
 		bch_hprint(target,	dc->writeback_rate_target << 9);
+		bch_hprint(proportional,dc->writeback_rate_proportional << 9);
+		bch_hprint(derivative,	dc->writeback_rate_derivative << 9);
+		bch_hprint(change,	dc->writeback_rate_change << 9);
+
+		next_io = div64_s64(dc->writeback_rate.next - local_clock(),
+				    NSEC_PER_MSEC);
 
 		return sprintf(buf,
-			       "rate:\t\t%u\n"
-			       "change:\t\t%i\n"
+			       "rate:\t\t%s/sec\n"
 			       "dirty:\t\t%s\n"
+			       "target:\t\t%s\n"
+			       "proportional:\t%s\n"
 			       "derivative:\t%s\n"
-			       "target:\t\t%s\n",
-			       dc->writeback_rate.rate,
-			       dc->writeback_rate_change,
-			       dirty, derivative, target);
+			       "change:\t\t%s/sec\n"
+			       "next io:\t%llims\n",
+			       rate, dirty, target, proportional,
+			       derivative, change, next_io);
 	}
 
 	sysfs_hprint(dirty_data,
@@ -189,6 +198,7 @@ STORE(__cached_dev)
 	struct kobj_uevent_env *env;
 
 #define d_strtoul(var)		sysfs_strtoul(var, dc->var)
+#define d_strtoul_nonzero(var)	sysfs_strtoul_clamp(var, dc->var, 1, INT_MAX)
 #define d_strtoi_h(var)		sysfs_hatoi(var, dc->var)
 
 	sysfs_strtoul(data_csum,	dc->disk.data_csum);
@@ -197,16 +207,15 @@ STORE(__cached_dev)
 	d_strtoul(writeback_metadata);
 	d_strtoul(writeback_running);
 	d_strtoul(writeback_delay);
-	sysfs_strtoul_clamp(writeback_rate,
-			    dc->writeback_rate.rate, 1, 1000000);
+
 	sysfs_strtoul_clamp(writeback_percent, dc->writeback_percent, 0, 40);
 
-	d_strtoul(writeback_rate_update_seconds);
+	sysfs_strtoul_clamp(writeback_rate,
+			    dc->writeback_rate.rate, 1, INT_MAX);
+
+	d_strtoul_nonzero(writeback_rate_update_seconds);
 	d_strtoul(writeback_rate_d_term);
-	d_strtoul(writeback_rate_p_term_inverse);
-	sysfs_strtoul_clamp(writeback_rate_p_term_inverse,
-			    dc->writeback_rate_p_term_inverse, 1, INT_MAX);
-	d_strtoul(writeback_rate_d_smooth);
+	d_strtoul_nonzero(writeback_rate_p_term_inverse);
 
 	d_strtoi_h(sequential_cutoff);
 	d_strtoi_h(readahead);
@@ -313,7 +322,6 @@ static struct attribute *bch_cached_dev_files[] = {
 	&sysfs_writeback_rate_update_seconds,
 	&sysfs_writeback_rate_d_term,
 	&sysfs_writeback_rate_p_term_inverse,
-	&sysfs_writeback_rate_d_smooth,
 	&sysfs_writeback_rate_debug,
 	&sysfs_dirty_data,
 	&sysfs_stripe_size,

drivers/md/bcache/util.c

@@ -209,7 +209,13 @@ uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done)
 {
 	uint64_t now = local_clock();
 
-	d->next += div_u64(done, d->rate);
+	d->next += div_u64(done * NSEC_PER_SEC, d->rate);
+
+	if (time_before64(now + NSEC_PER_SEC, d->next))
+		d->next = now + NSEC_PER_SEC;
+
+	if (time_after64(now - NSEC_PER_SEC * 2, d->next))
+		d->next = now - NSEC_PER_SEC * 2;
 
 	return time_after64(d->next, now)
 		? div_u64(d->next - now, NSEC_PER_SEC / HZ)

drivers/md/bcache/writeback.c

@@ -30,38 +30,40 @@ static void __update_writeback_rate(struct cached_dev *dc)
 
 	/* PD controller */
 
-	int change = 0;
-	int64_t error;
 	int64_t dirty = bcache_dev_sectors_dirty(&dc->disk);
 	int64_t derivative = dirty - dc->disk.sectors_dirty_last;
+	int64_t proportional = dirty - target;
+	int64_t change;
 
 	dc->disk.sectors_dirty_last = dirty;
 
-	derivative *= dc->writeback_rate_d_term;
-	derivative = clamp(derivative, -dirty, dirty);
+	/* Scale to sectors per second */
 
-	derivative = ewma_add(dc->disk.sectors_dirty_derivative, derivative,
-			      dc->writeback_rate_d_smooth, 0);
+	proportional *= dc->writeback_rate_update_seconds;
+	proportional = div_s64(proportional, dc->writeback_rate_p_term_inverse);
 
-	/* Avoid divide by zero */
-	if (!target)
-		goto out;
+	derivative = div_s64(derivative, dc->writeback_rate_update_seconds);
 
-	error = div64_s64((dirty + derivative - target) << 8, target);
+	derivative = ewma_add(dc->disk.sectors_dirty_derivative, derivative,
+			      (dc->writeback_rate_d_term /
+			       dc->writeback_rate_update_seconds) ?: 1, 0);
+
+	derivative *= dc->writeback_rate_d_term;
+	derivative = div_s64(derivative, dc->writeback_rate_p_term_inverse);
 
-	change = div_s64((dc->writeback_rate.rate * error) >> 8,
-			 dc->writeback_rate_p_term_inverse);
+	change = proportional + derivative;
 
 	/* Don't increase writeback rate if the device isn't keeping up */
 	if (change > 0 &&
 	    time_after64(local_clock(),
-			 dc->writeback_rate.next + 10 * NSEC_PER_MSEC))
+			 dc->writeback_rate.next + NSEC_PER_MSEC))
 		change = 0;
 
 	dc->writeback_rate.rate =
-		clamp_t(int64_t, dc->writeback_rate.rate + change,
+		clamp_t(int64_t, (int64_t) dc->writeback_rate.rate + change,
 			1, NSEC_PER_MSEC);
-out:
+
+	dc->writeback_rate_proportional = proportional;
 	dc->writeback_rate_derivative = derivative;
 	dc->writeback_rate_change = change;
 	dc->writeback_rate_target = target;
@@ -87,15 +89,11 @@ static void update_writeback_rate(struct work_struct *work)
 
 static unsigned writeback_delay(struct cached_dev *dc, unsigned sectors)
 {
-	uint64_t ret;
-
 	if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) ||
 	    !dc->writeback_percent)
 		return 0;
 
-	ret = bch_next_delay(&dc->writeback_rate, sectors * 10000000ULL);
-
-	return min_t(uint64_t, ret, HZ);
+	return bch_next_delay(&dc->writeback_rate, sectors);
 }
 
 struct dirty_io {
@@ -476,6 +474,8 @@ void bch_sectors_dirty_init(struct cached_dev *dc)
 
 	bch_btree_map_keys(&op.op, dc->disk.c, &KEY(op.inode, 0, 0),
 			   sectors_dirty_init_fn, 0);
+
+	dc->disk.sectors_dirty_last = bcache_dev_sectors_dirty(&dc->disk);
 }
 
 int bch_cached_dev_writeback_init(struct cached_dev *dc)
@@ -490,10 +490,9 @@ int bch_cached_dev_writeback_init(struct cached_dev *dc)
 	dc->writeback_delay		= 30;
 	dc->writeback_rate.rate		= 1024;
 
-	dc->writeback_rate_update_seconds = 30;
-	dc->writeback_rate_d_term	= 16;
-	dc->writeback_rate_p_term_inverse = 64;
-	dc->writeback_rate_d_smooth	= 8;
+	dc->writeback_rate_update_seconds = 5;
+	dc->writeback_rate_d_term	= 30;
+	dc->writeback_rate_p_term_inverse = 6000;
 
 	dc->writeback_thread = kthread_create(bch_writeback_thread, dc,
 					      "bcache_writeback");