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Commit 16749c23 authored by Kent Overstreet's avatar Kent Overstreet
Browse files

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: default avatarKent Overstreet <kmo@daterainc.com>
parent 6d3d1a9c

drivers/md/bcache/bcache.h

+3 −3
Original line number Diff line number Diff line
@@ -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

+29 −21
Original line number Diff line number Diff line
@@ -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

+7 −1
Original line number Diff line number Diff line
@@ -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

+23 −24
Original line number Diff line number Diff line
@@ -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");