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

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Message-ID: <20200520143627.047a7eee@hermes.lan>
Date:   Wed, 20 May 2020 14:36:27 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     David Miller <davem@...emloft.net>, 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 Wed, 20 May 2020 21:37:11 +0200
Thomas Gleixner <tglx@...utronix.de> wrote:

> David Miller <davem@...emloft.net> writes:
> > From: Thomas Gleixner <tglx@...utronix.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.