Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of a seqcount

Thomas Gleixner
2020-05-20T19:37:37Z

David Miller <davem@davemloft.net> writes:
> From: Thomas Gleixner <tglx@linutronix.de>
> Date: Wed, 20 May 2020 01:42:30 +0200
>>> Please try, it isn't that hard..
>>>
>>> # time for ((i=0;i<1000;i++)); do ip li add dev dummy$i type dummy; done
>>>
>>> real 0m17.002s
>>> user 0m1.064s
>>> sys 0m0.375s
>>
>> And that solves the incorrectness of the current code in which way?
>
> You mentioned that there wasn't a test case, he gave you one to try.

If it makes you happy to compare incorrrect code with correct code, here
you go:

5 runs of 1000 device add, 1000 device rename and 1000 device del

CONFIG_PREEMPT_NONE=y

Base rwsem
add 0:05.01 0:05.28
0:05.93 0:06.11
0:06.52 0:06.26
0:06.06 0:05.74
0:05.71 0:06.07

rename 0:32.57 0:33.04
0:32.91 0:32.45
0:32.72 0:32.53
0:39.65 0:34.18
0:34.52 0:32.50

delete 3:48.65 3:48.91
3:49.66 3:49.13
3:45.29 3:48.26
3:47.56 3:46.60
3:50.01 3:48.06

-------------------------

CONFIG_PREEMPT_VOLUNTARY=y

Base rwsem
add 0:06.80 0:06.42
0:04.77 0:05.03
0:05.74 0:04.62
0:05.87 0:04.34
0:04.20 0:04.12

rename 0:33.33 0:42.02
0:42.36 0:32.55
0:39.58 0:31.60
0:33.69 0:35.08
0:34.24 0:33.97

delete 3:47.82 3:44.00
3:47.42 3:51.00
3:48.52 3:48.88
3:48.50 3:48.09
3:50.03 3:46.56

-------------------------

CONFIG_PREEMPT=y

Base rwsem

add 0:07.89 0:07.72
0:07.25 0:06.72
0:07.42 0:06.51
0:06.92 0:06.38
0:06.20 0:06.72

rename 0:41.77 0:32.39
0:44.29 0:33.29
0:36.19 0:34.86
0:33.19 0:35.06
0:37.00 0:34.78

delete 2:36.96 2:39.97
2:37.80 2:42.19
2:44.66 2:48.40
2:39.75 2:41.02
2:40.77 2:38.36

The runtime variation is rather large and when running the same in a VM
I got complete random numbers for both base and rwsem. The most amazing
was delete where the time varies from 30s to 6m20s.

Btw, Sebastian noticed that rename spams dmesg:

netdev_info(dev, "renamed from %s\n", oldname);

which eats about 50% of the Rename run time.

Base netdev_info() removed

Rename 0:34.84 0:17.48

That number at least makes tons of sense

Thanks,

tglx