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 <[e-mail]>
Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/mean_and_variance.c

···
  * 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;
···
 	// 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);
 

fs/bcachefs/mean_and_variance.h

···
 
 /* expontentially weighted variant */
 struct mean_and_variance_weighted {
-	bool	init;
-	u8	weight;	/* base 2 logarithim */
 	s64	mean;
 	u64	variance;
 };
···
 u64 mean_and_variance_get_variance(struct mean_and_variance s1);
 u32 mean_and_variance_get_stddev(struct mean_and_variance s);
 
-void mean_and_variance_weighted_update(struct mean_and_variance_weighted *s, s64 v);
+void mean_and_variance_weighted_update(struct mean_and_variance_weighted *s,
+		s64 v, bool initted, u8 weight);
 
-s64 mean_and_variance_weighted_get_mean(struct mean_and_variance_weighted s);
-u64 mean_and_variance_weighted_get_variance(struct mean_and_variance_weighted s);
-u32 mean_and_variance_weighted_get_stddev(struct mean_and_variance_weighted s);
+s64 mean_and_variance_weighted_get_mean(struct mean_and_variance_weighted s,
+		u8 weight);
+u64 mean_and_variance_weighted_get_variance(struct mean_and_variance_weighted s,
+		u8 weight);
+u32 mean_and_variance_weighted_get_stddev(struct mean_and_variance_weighted s,
+		u8 weight);
 
 #endif // MEAN_AND_VAIRANCE_H_

fs/bcachefs/mean_and_variance_test.c

···
 
 static void mean_and_variance_weighted_test(struct kunit *test)
 {
-	struct mean_and_variance_weighted s = { .weight = 2 };
+	struct mean_and_variance_weighted s = { };
 
-	mean_and_variance_weighted_update(&s, 10);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 10);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 0);
+	mean_and_variance_weighted_update(&s, 10, false, 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 2), 10);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 2), 0);
 
-	mean_and_variance_weighted_update(&s, 20);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 12);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 18);
+	mean_and_variance_weighted_update(&s, 20, true, 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 2), 12);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 2), 18);
 
-	mean_and_variance_weighted_update(&s, 30);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 16);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 72);
+	mean_and_variance_weighted_update(&s, 30, true, 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 2), 16);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 2), 72);
 
-	s = (struct mean_and_variance_weighted) { .weight = 2 };
+	s = (struct mean_and_variance_weighted) { };
 
-	mean_and_variance_weighted_update(&s, -10);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -10);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 0);
+	mean_and_variance_weighted_update(&s, -10, false, 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 2), -10);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 2), 0);
 
-	mean_and_variance_weighted_update(&s, -20);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -12);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 18);
+	mean_and_variance_weighted_update(&s, -20, true, 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 2), -12);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 2), 18);
 
-	mean_and_variance_weighted_update(&s, -30);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -16);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 72);
+	mean_and_variance_weighted_update(&s, -30, true, 2);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 2), -16);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 2), 72);
 }
 
 static void mean_and_variance_weighted_advanced_test(struct kunit *test)
 {
-	struct mean_and_variance_weighted s = { .weight = 8 };
+	struct mean_and_variance_weighted s = { };
+	bool initted = false;
 	s64 i;
 
-	for (i = 10; i <= 100; i += 10)
-		mean_and_variance_weighted_update(&s, i);
+	for (i = 10; i <= 100; i += 10) {
+		mean_and_variance_weighted_update(&s, i, initted, 8);
+		initted = true;
+	}
 
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), 11);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 107);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 8), 11);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 8), 107);
 
-	s = (struct mean_and_variance_weighted) { .weight = 8 };
+	s = (struct mean_and_variance_weighted) { };
+	initted = false;
 
-	for (i = -10; i >= -100; i -= 10)
-		mean_and_variance_weighted_update(&s, i);
+	for (i = -10; i >= -100; i -= 10) {
+		mean_and_variance_weighted_update(&s, i, initted, 8);
+		initted = true;
+	}
 
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s), -11);
-	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s), 107);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(s, 8), -11);
+	KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_variance(s, 8), 107);
 }
 
 static void do_mean_and_variance_test(struct kunit *test,
···
 				      s64 *weighted_stddev)
 {
 	struct mean_and_variance mv = {};
-	struct mean_and_variance_weighted vw = { .weight = weight };
+	struct mean_and_variance_weighted vw = { };
 
 	for (unsigned i = 0; i < initial_n; i++) {
 		mean_and_variance_update(&mv, initial_value);
-		mean_and_variance_weighted_update(&vw, initial_value);
+		mean_and_variance_weighted_update(&vw, initial_value, false, weight);
 
 		KUNIT_EXPECT_EQ(test, mean_and_variance_get_mean(mv),		initial_value);
 		KUNIT_EXPECT_EQ(test, mean_and_variance_get_stddev(mv),		0);
-		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(vw),	initial_value);
-		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_stddev(vw),0);
+		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(vw, weight),	initial_value);
+		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_stddev(vw, weight),0);
 	}
 
 	for (unsigned i = 0; i < n; i++) {
 		mean_and_variance_update(&mv, data[i]);
-		mean_and_variance_weighted_update(&vw, data[i]);
+		mean_and_variance_weighted_update(&vw, data[i], true, weight);
 
 		KUNIT_EXPECT_EQ(test, mean_and_variance_get_mean(mv),		mean[i]);
 		KUNIT_EXPECT_EQ(test, mean_and_variance_get_stddev(mv),		stddev[i]);
-		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(vw),	weighted_mean[i]);
-		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_stddev(vw),weighted_stddev[i]);
+		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_mean(vw, weight),	weighted_mean[i]);
+		KUNIT_EXPECT_EQ(test, mean_and_variance_weighted_get_stddev(vw, weight),weighted_stddev[i]);
 	}
 
 	KUNIT_EXPECT_EQ(test, mv.n, initial_n + n);

fs/bcachefs/time_stats.c

···
 					      u64 start, u64 end)
 {
 	u64 duration, freq;
+	bool initted = stats->last_event != 0;
 
 	if (time_after64(end, start)) {
 		duration = end - start;
 		mean_and_variance_update(&stats->duration_stats, duration);
-		mean_and_variance_weighted_update(&stats->duration_stats_weighted, duration);
+		mean_and_variance_weighted_update(&stats->duration_stats_weighted,
+				duration, initted, TIME_STATS_MV_WEIGHT);
 		stats->max_duration = max(stats->max_duration, duration);
 		stats->min_duration = min(stats->min_duration, duration);
 		stats->total_duration += duration;
···
 	if (stats->last_event && time_after64(end, stats->last_event)) {
 		freq = end - stats->last_event;
 		mean_and_variance_update(&stats->freq_stats, freq);
-		mean_and_variance_weighted_update(&stats->freq_stats_weighted, freq);
+		mean_and_variance_weighted_update(&stats->freq_stats_weighted,
+				freq, initted, TIME_STATS_MV_WEIGHT);
 		stats->max_freq = max(stats->max_freq, freq);
 		stats->min_freq = min(stats->min_freq, freq);
 	}
···
 {
 	unsigned long flags;
 
-	WARN_ONCE(!stats->duration_stats_weighted.weight ||
-		  !stats->freq_stats_weighted.weight,
-		  "uninitialized bch2_time_stats");
-
 	if (!stats->buffer) {
 		spin_lock_irqsave(&stats->lock, flags);
 		time_stats_update_one(stats, start, end);
 
-		if (mean_and_variance_weighted_get_mean(stats->freq_stats_weighted) < 32 &&
+		if (mean_and_variance_weighted_get_mean(stats->freq_stats_weighted, TIME_STATS_MV_WEIGHT) < 32 &&
 		    stats->duration_stats.n > 1024)
 			stats->buffer =
 				alloc_percpu_gfp(struct time_stat_buffer,
···
 void bch2_time_stats_init(struct bch2_time_stats *stats)
 {
 	memset(stats, 0, sizeof(*stats));
-	stats->duration_stats_weighted.weight = 8;
-	stats->freq_stats_weighted.weight = 8;
 	stats->min_duration = U64_MAX;
 	stats->min_freq = U64_MAX;
 	spin_lock_init(&stats->lock);

fs/bcachefs/time_stats.h

···
 	struct quantiles quantiles;
 
 	struct mean_and_variance	  duration_stats;
-	struct mean_and_variance_weighted duration_stats_weighted;
 	struct mean_and_variance	  freq_stats;
+
+/* default weight for weighted mean and variance calculations */
+#define TIME_STATS_MV_WEIGHT	8
+
+	struct mean_and_variance_weighted duration_stats_weighted;
 	struct mean_and_variance_weighted freq_stats_weighted;
 	struct time_stat_buffer __percpu *buffer;
 };

fs/bcachefs/util.c

···
 	prt_tab(out);
 	bch2_pr_time_units_aligned(out, d_mean);
 	prt_tab(out);
-	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_mean(stats->duration_stats_weighted));
+	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_mean(stats->duration_stats_weighted, TIME_STATS_MV_WEIGHT));
 	prt_newline(out);
 
 	prt_printf(out, "stddev:");
 	prt_tab(out);
 	bch2_pr_time_units_aligned(out, d_stddev);
 	prt_tab(out);
-	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_stddev(stats->duration_stats_weighted));
+	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_stddev(stats->duration_stats_weighted, TIME_STATS_MV_WEIGHT));
 
 	printbuf_indent_sub(out, 2);
 	prt_newline(out);
···
 	prt_tab(out);
 	bch2_pr_time_units_aligned(out, f_mean);
 	prt_tab(out);
-	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_mean(stats->freq_stats_weighted));
+	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_mean(stats->freq_stats_weighted, TIME_STATS_MV_WEIGHT));
 	prt_newline(out);
 
 	prt_printf(out, "stddev:");
 	prt_tab(out);
 	bch2_pr_time_units_aligned(out, f_stddev);
 	prt_tab(out);
-	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_stddev(stats->freq_stats_weighted));
+	bch2_pr_time_units_aligned(out, mean_and_variance_weighted_get_stddev(stats->freq_stats_weighted, TIME_STATS_MV_WEIGHT));
 
 	printbuf_indent_sub(out, 2);
 	prt_newline(out);