bcachefs: mean_and_variance: put struct mean_and_variance_weighted on a diet

The only caller of this code (time_stats) always knows the weights and
whether or not any information has been collected.  Pass this
information into the mean and variance code so that it doesn't have to
store that information.  This reduces the structure size from 24 to 16
bytes, which shrinks each time_stats counter to 192 bytes from 208.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 18 insertions, 10 deletions

diff --git a/fs/bcachefs/mean_and_variance.c b/fs/bcachefs/mean_and_variance.c
index bf0ef668fd38..0ea9f30803a2 100644
--- a/fs/bcachefs/mean_and_variance.c
+++ b/fs/bcachefs/mean_and_variance.c
@@ -103,14 +103,17 @@ EXPORT_SYMBOL_GPL(mean_and_variance_get_stddev);
  * mean_and_variance_weighted_update() - exponentially weighted variant of mean_and_variance_update()
  * @s: mean and variance number of samples and their sums
  * @x: new value to include in the &mean_and_variance_weighted
+ * @initted: caller must track whether this is the first use or not
+ * @weight: ewma weight
  *
  * see linked pdf: function derived from equations 140-143 where alpha = 2^w.
  * values are stored bitshifted for performance and added precision.
  */
-void mean_and_variance_weighted_update(struct mean_and_variance_weighted *s, s64 x)
+void mean_and_variance_weighted_update(struct mean_and_variance_weighted *s,
+		s64 x, bool initted, u8 weight)
 {
 	// previous weighted variance.
-	u8 w		= s->weight;
+	u8 w		= weight;
 	u64 var_w0	= s->variance;
 	// new value weighted.
 	s64 x_w		= x << w;
@@ -119,45 +122,50 @@ void mean_and_variance_weighted_update(struct mean_and_variance_weighted *s, s64
 	// new mean weighted.
 	s64 u_w1	= s->mean + diff;
 
-	if (!s->init) {
+	if (!initted) {
 		s->mean = x_w;
 		s->variance = 0;
 	} else {
 		s->mean = u_w1;
 		s->variance = ((var_w0 << w) - var_w0 + ((diff_w * (x_w - u_w1)) >> w)) >> w;
 	}
-	s->init = true;
 }
 EXPORT_SYMBOL_GPL(mean_and_variance_weighted_update);
 
 /**
  * mean_and_variance_weighted_get_mean() - get mean from @s
  * @s: mean and variance number of samples and their sums
+ * @weight: ewma weight
  */
-s64 mean_and_variance_weighted_get_mean(struct mean_and_variance_weighted s)
+s64 mean_and_variance_weighted_get_mean(struct mean_and_variance_weighted s,
+		u8 weight)
 {
-	return fast_divpow2(s.mean, s.weight);
+	return fast_divpow2(s.mean, weight);
 }
 EXPORT_SYMBOL_GPL(mean_and_variance_weighted_get_mean);
 
 /**
  * mean_and_variance_weighted_get_variance() -- get variance from @s
  * @s: mean and variance number of samples and their sums
+ * @weight: ewma weight
  */
-u64 mean_and_variance_weighted_get_variance(struct mean_and_variance_weighted s)
+u64 mean_and_variance_weighted_get_variance(struct mean_and_variance_weighted s,
+		u8 weight)
 {
 	// always positive don't need fast divpow2
-	return s.variance >> s.weight;
+	return s.variance >> weight;
 }
 EXPORT_SYMBOL_GPL(mean_and_variance_weighted_get_variance);
 
 /**
  * mean_and_variance_weighted_get_stddev() - get standard deviation from @s
  * @s: mean and variance number of samples and their sums
+ * @weight: ewma weight
  */
-u32 mean_and_variance_weighted_get_stddev(struct mean_and_variance_weighted s)
+u32 mean_and_variance_weighted_get_stddev(struct mean_and_variance_weighted s,
+		u8 weight)
 {
-	return int_sqrt64(mean_and_variance_weighted_get_variance(s));
+	return int_sqrt64(mean_and_variance_weighted_get_variance(s, weight));
 }
 EXPORT_SYMBOL_GPL(mean_and_variance_weighted_get_stddev);