mac80211: reduce calculation costs of EWMA

thuehn · jmberg-intel · commit ade6d4a2ec57 · 2015-04-01T20:44:33.000+02:00
This patch reduces the calculation costs of the EWMA macro from
"2x multiplication and 1 addition" down to "1x multiplication and
2x additions". This slightly improves performance depending on the
CPU architecture.

Signed-off-by: Thomas Huehn &lt;thomas@net.t-labs.tu-berlin.de&gt;
Acked-by: Felix Fietkau &lt;nbd@openwrt.org&gt;
Signed-off-by: Johannes Berg &lt;johannes.berg@intel.com&gt;
diff --git a/net/mac80211/rc80211_minstrel.h b/net/mac80211/rc80211_minstrel.h
@@ -27,7 +27,12 @@
 static inline int
 minstrel_ewma(int old, int new, int weight)
 {
-	return (new * (EWMA_DIV - weight) + old * weight) / EWMA_DIV;
+	int diff, incr;
+
+	diff = new - old;
+	incr = (EWMA_DIV - weight) * diff / EWMA_DIV;
+
+	return old + incr;
 }
 
 struct minstrel_rate_stats {

Original file line number	Diff line number	Diff line change
`@@ -27,7 +27,12 @@`
`27`	`27`	`static inline int`
`28`	`28`	`minstrel_ewma(int old, int new, int weight)`
`29`	`29`	`{`
`30`		`- return (new * (EWMA_DIV - weight) + old * weight) / EWMA_DIV;`
	`30`	`+ int diff, incr;`
	`31`	`+`
	`32`	`+ diff = new - old;`
	`33`	`+ incr = (EWMA_DIV - weight) * diff / EWMA_DIV;`
	`34`	`+`
	`35`	`+ return old + incr;`
`31`	`36`	`}`
`32`	`37`
`33`	`38`	`struct minstrel_rate_stats {`