tcp, util: Add function for scaling to linearly interpolated factor, use it

Right now, the only need for this kind of function comes from
tcp_get_sndbuf(), which calculates the amount of sending buffer we
want to use depending on its own size: we want to use more of it
if it's smaller, as bookkeeping overhead is usually lower and we rely
on auto-tuning there, and use less of it when it's bigger.

For this purpose, the new function is overly generic: @x is the same
as @y, that is, we want to use more or less of the buffer depending
on the size of the buffer itself.

However, an upcoming change will need that generality, as we'll want
to scale the amount of sending buffer we use depending on another
(scaled) factor.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

3 files changed, 40 insertions, 5 deletions

diff --git a/tcp.c b/tcp.c
index bb661ee..026546a 100644
--- a/tcp.c
+++ b/tcp.c
@@ -788,11 +788,7 @@ static void tcp_get_sndbuf(struct tcp_tap_conn *conn)
 		return;
 	}
 
-	v = sndbuf;
-	if (v >= SNDBUF_BIG)
-		v /= 2;
-	else if (v > SNDBUF_SMALL)
-		v -= v * (v - SNDBUF_SMALL) / (SNDBUF_BIG - SNDBUF_SMALL) / 2;
+	v = clamped_scale(sndbuf, sndbuf, SNDBUF_SMALL, SNDBUF_BIG, 50);
 
 	SNDBUF_SET(conn, MIN(INT_MAX, v));
 }
diff --git a/util.c b/util.c
index f32c9cb..2232a24 100644
--- a/util.c
+++ b/util.c
@@ -1223,3 +1223,41 @@ void fsync_pcap_and_log(void)
 	if (log_file != -1)
 		(void)fsync(log_file);
 }
+
+/**
+ * clamped_scale() - Scale @x from 100% to f% depending on @y's value
+ * @x:		Value to scale
+ * @y:		Value determining scaling
+ * @lo:		Lower bound for @y (start of y-axis slope)
+ * @hi:		Upper bound for @y (end of y-axis slope)
+ * @f:		Scaling factor, percent (might be less or more than 100)
+ *
+ * Return: @x scaled by @f * linear interpolation of @y between @lo and @hi
+ *
+ * In pictures:
+ *
+ *                f % -> ,----   * If @y < lo (for example, @y is y0), return @x
+ *                      /|   |
+ *                     / |   |   * If @lo < @y < @hi (for example, @y is y1),
+ *                    /  |   |     return @x scaled by a factor linearly
+ * (100 + f) / 2 % ->/   |   |     interpolated between 100% and f% depending on
+ *                  /|   |   |     @y's position between @lo (100%) and @hi (f%)
+ *                 / |   |   |
+ *                /  |   |   |   * If @y > @hi (for example, @y is y2), return
+ * 100 % -> -----'   |   |   |     @x * @f / 100
+ *           |   |   |   |   |
+ *          y0  lo  y1  hi  y2   Example: @f = 150, @lo = 10, @hi = 20, @y = 15,
+ *                                        @x = 1000
+ *                                        -> interpolated factor is 125%
+ *                                        -> return 1250
+ */
+long clamped_scale(long x, long y, long lo, long hi, long f)
+{
+	if (y < lo)
+		return x;
+
+	if (y > hi)
+		return x * f / 100;
+
+	return x - (x * (y - lo) / (hi - lo)) * (100 - f) / 100;
+}
diff --git a/util.h b/util.h
index 17f5ae0..744880b 100644
--- a/util.h
+++ b/util.h
@@ -242,6 +242,7 @@ int read_remainder(int fd, const struct iovec *iov, size_t cnt, size_t skip);
 void close_open_files(int argc, char **argv);
 bool snprintf_check(char *str, size_t size, const char *format, ...);
 void fsync_pcap_and_log(void);
+long clamped_scale(long x, long y, long lo, long hi, long f);
 
 /**
  * af_name() - Return name of an address family