When playing with Linux Bridge in Ubuntu 18.04(Travis CI), I noticed
the default value of multicast_startup_query_interval option is 3124 while
RHEL/Fedora is 3125:
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| fge@ubt1804:~$ sudo ip link add br0 type bridge
fge@ubt1804:~$ cat /sys/class/net/br0/bridge/multicast_startup_query_interval
3124
|
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| [fge@el8 ~]$ sudo ip link add br0 type bridge
[fge@el8 ~]$ cat /sys/class/net/br0/bridge/multicast_startup_query_interval
3125
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After debug, I confirmed this is caused by integer round up at two places:
- Default value setter:
125 * HZ / 4 jiffies_to_clock_t()
The default value of multicast_startup_query_interval in kernel jiffies:
- Ubuntu 18.04, the kernel is holding
7812 rounded from 7812.5. - RHEL 8.3, the kernel is holding
31250, no round up.
When showing via sysfs, kernel use jiffies_to_clock_t(), both 250HZ and
1000HZ kernel uses:
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| NSEC_PER_SEC = 1000000000L
clock_t = jiffies * ((NSEC_PER_SEC + hz / 2) / HZ ) / (NSEC_PER_SEC / USER_HZ))
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Above formation will generate 3124 with 7812 jiffies on 250HZ kernel and
100HZ(hard coded) USER_HZ.
Below python script could check the maximum deviation caused by round up
of jiffies_to_clock_t() in 250HZ kernel. I only found the maximum deviation
as 1 for a very long exhaustion.
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| #!/usr/bin/python3
NSEC_PER_SEC = 10 ** 9
MAX_U64 = 2 ** 64
def jiffies_to_clock_t(user_hz, hz, jiffies):
tick_nsec = int((NSEC_PER_SEC + int(hz / 2)) / hz)
if (tick_nsec % (NSEC_PER_SEC / user_hz)) == 0:
if hz < user_hz:
return int(jiffies * (user_hz / hz))
else:
return int(jiffies / (hz / user_hz))
else:
return int(jiffies * tick_nsec / (NSEC_PER_SEC / user_hz))
def clock_t_to_jiffies(user_hz, hz, clock):
if hz % user_hz == 0:
if clock >= MAX_U64 / (hz / user_hz):
return MAX_U64
return int(clock * (hz / user_hz))
if clock >= MAX_U64 / hz * user_hz:
return MAX_U64
return int(clock * hz / user_hz)
max_deviation = 0
for original_num in range(0, 2 ** 32):
if original_num % 100000 == 0:
print(
f"Tested {original_num} and found max_deviation: {max_deviation}"
)
user_hz = 100
hz = 250
new_num = jiffies_to_clock_t(
user_hz, hz, clock_t_to_jiffies(user_hz, hz, original_num)
)
a = abs(original_num - new_num)
if a > max_deviation:
max_deviation = a
print(f"{original_num}: {max_deviation}")
print(max_deviation)
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