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[iproute2] tc: Fix rounding in tc_calc_xmittime and tc_calc_xmitsize.

Message ID 85371219-A098-4873-B3B9-0E881E812F2A@8x8.com (mailing list archive)
State New
Delegated to: David Ahern
Headers show
Series [iproute2] tc: Fix rounding in tc_calc_xmittime and tc_calc_xmitsize. | expand

Checks

Context Check Description
netdev/tree_selection success Not a local patch

Commit Message

Jonathan Lennox Feb. 5, 2025, 9:16 p.m. UTC 
The logic in tc that converts between sizes and times for a given rate (the
functions tc_calc_xmittime and tc_calc_xmitsize) suffers from double rounding,
with intermediate values getting cast to unsigned int.

As a result, for example, on my test system (where tick_in_usec=15.625,
clock_factor=1, and hz=1000000000) for a bitrate of 1Gbps, all tc htb burst
values between 0 and 999 get encoded as 0; all values between 1000 and 1999
get encoded as 15 (equivalent to 960 bytes); all values between 2000 and 2999
as 31 (1984 bytes); etc.

The attached patch changes this so these calculations are done entirely in
floating-point, and only rounded to integer values when the value is returned.
It also changes tc_calc_xmittime to round its calculated value up, rather than
down, to ensure that the calculated time is actually sufficient for the requested
size.

This is a userspace-only fix to tc; no kernel changes are necessary.

(Please let me know if anything is wrong with this patch, this is my first
time submitting to any Linux kernel mailing lists.)

---
tc/tc_core.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff mbox series

Patch

diff --git a/tc/tc_core.c b/tc/tc_core.c
index 37547e9b..ba8d5bf0 100644
--- a/tc/tc_core.c
+++ b/tc/tc_core.c
@@ -23,11 +23,16 @@ 
static double tick_in_usec = 1;
static double clock_factor = 1;

-static unsigned int tc_core_time2tick(unsigned int time)
+static double tc_core_time2tick_d(double time)
{
	return time * tick_in_usec;
}

+static double tc_core_tick2time_d(double tick)
+{
+	return tick / tick_in_usec;
+}
+
unsigned int tc_core_tick2time(unsigned int tick)
{
	return tick / tick_in_usec;
@@ -45,12 +50,12 @@  unsigned int tc_core_ktime2time(unsigned int ktime)

unsigned int tc_calc_xmittime(__u64 rate, unsigned int size)
{
-	return tc_core_time2tick(TIME_UNITS_PER_SEC*((double)size/(double)rate));
+	return ceil(tc_core_time2tick_d(TIME_UNITS_PER_SEC*((double)size/(double)rate)));
}

unsigned int tc_calc_xmitsize(__u64 rate, unsigned int ticks)
{
-	return ((double)rate*tc_core_tick2time(ticks))/TIME_UNITS_PER_SEC;
+	return ((double)rate*tc_core_tick2time_d(ticks))/TIME_UNITS_PER_SEC;
}

/*