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

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Date:   Tue, 19 May 2020 21:36:37 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>, tglx@...utronix.de,
        a.darwish@...utronix.de, peterz@...radead.org, mingo@...hat.com,
        will@...nel.org, paulmck@...nel.org, bigeasy@...utronix.de,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org, kuba@...nel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of
 a seqcount

On Tue, 19 May 2020 20:18:19 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On 5/19/20 7:57 PM, David Miller wrote:
> > From: Thomas Gleixner <tglx@...utronix.de>
> > Date: Wed, 20 May 2020 01:42:30 +0200
> >   
> >> Stephen Hemminger <stephen@...workplumber.org> writes:  
> >>> On Wed, 20 May 2020 00:23:48 +0200
> >>> Thomas Gleixner <tglx@...utronix.de> wrote:  
> >>>> No. We did not. -ENOTESTCASE  
> >>>
> >>> Please try, it isn't that hard..
> >>>
> >>> # time for ((i=0;i<1000;i++)); do ip li add dev dummy$i type dummy; done
> >>>
> >>> real	0m17.002s
> >>> user	0m1.064s
> >>> sys	0m0.375s  
> >>
> >> And that solves the incorrectness of the current code in which way?  
> > 
> > You mentioned that there wasn't a test case, he gave you one to try.
> >   
> 
> I do not think this would ever use device rename, nor netdev_get_name()
> 
> None of this stuff is fast path really.
> 
> # time for ((i=1;i<1000;i++)); do ip li add dev dummy$i type dummy; done
> 
> real	0m1.127s
> user	0m0.270s
> sys	0m1.039s

Your right it is a weak test, and most of the overhead is in the syscall
and all netlink events that happen.

It does end up looking up the new name, so would exercise that.
Better test is to use %d syntax or create 1000 dummy's then rename every one.

This is more of a stress test
# for ((i=0;i<1000;i++)); do echo link add dev dummy%d type dummy; done | time ip -batch -
0.00user 0.29system 0:02.11elapsed 13%CPU (0avgtext+0avgdata 2544maxresident)k
0inputs+0outputs (0major+148minor)pagefaults 0swaps

# for ((i=999;i>=0;i--)); do echo link set dummy$i name dummy$((i+1)); done | time ip -batch -
0.00user 0.26system 0:54.98elapsed 0%CPU (0avgtext+0avgdata 2508maxresident)k
0inputs+0outputs (0major+145minor)pagefaults 0swaps