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

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Date:   Tue, 19 May 2020 17:06:37 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH v1 01/25] net: core: device_rename: Use rwsem instead of
 a seqcount

On Wed, 20 May 2020 01:42:30 +0200
Thomas Gleixner <tglx@...utronix.de> wrote:

> 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?

Agree that the current code is has evolved over time to a state where it is not
correct in the case of Preempt-RT. The motivation for the changes to seqcount
goes back many years when there were ISP's that were concerned about scaling of tunnels, vlans etc.

Is it too much to ask for a simple before/after test of your patch as part 
of the submission. You probably measure latency changes to the nanosecond.

Getting it correct without causing user complaints.