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

Stephen Hemminger
2020-05-20T21:36:38Z

On Wed, 20 May 2020 21:37:11 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> 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

Looks good thanks for following through.