[PATCH v2 1/9] tcp, util: Add function for scaling to linearly interpolated factor, use it

[Stefano Brivio <sbrivio@redhat.com>]

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 and its name is
a mouthful: @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.

While at it, now that we have this new function, which makes it simple
to specify a precise usage factor, change the amount of sending buffer
we want to use at and above 4 MiB: 75% looks perfectly safe.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c  |  8 ++------
 util.c | 38 ++++++++++++++++++++++++++++++++++++++
 util.h |  1 +
 3 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/tcp.c b/tcp.c
index bb661ee..37012cc 100644
--- a/tcp.c
+++ b/tcp.c
@@ -773,7 +773,7 @@ static void tcp_rtt_dst_check(const struct tcp_tap_conn *conn,
 }
 
 /**
- * tcp_get_sndbuf() - Get, scale SO_SNDBUF between thresholds (1 to 0.5 usage)
+ * tcp_get_sndbuf() - Get, scale SO_SNDBUF between thresholds (1 to 0.75 usage)
  * @conn:	Connection pointer
  */
 static void tcp_get_sndbuf(struct tcp_tap_conn *conn)
@@ -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 = scale_x_to_y_slope(sndbuf, sndbuf, SNDBUF_SMALL, SNDBUF_BIG, 75);
 
 	SNDBUF_SET(conn, MIN(INT_MAX, v));
 }
diff --git a/util.c b/util.c
index f32c9cb..ff0ba01 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);
 }
+
+/**
+ * scale_x_to_y_slope() - 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
+ *
+ * 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 scale_x_to_y_slope(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..ec75453 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 scale_x_to_y_slope(long x, long y, long lo, long hi, long f);
 
 /**
  * af_name() - Return name of an address family
-- 
2.43.0