netdev - Re: [PATCH net-next] net-procfs: use xarray iterator to implement /proc/net/dev

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Date: Wed, 7 Feb 2024 09:55:49 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Paolo Abeni
 <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next] net-procfs: use xarray iterator to implement
 /proc/net/dev

On Wed,  7 Feb 2024 16:53:18 +0000 Eric Dumazet wrote:
> In commit 759ab1edb56c ("net: store netdevs in an xarray")
> Jakub added net->dev_by_index to map ifindex to netdevices.
> 
> We can get rid of the old hash table (net->dev_index_head),
> one patch at a time, if performance is acceptable.

FWIW there was a basic benchmark result in that commit:

     #devs | hash |  xa  | delta
        2  | 18.3 | 20.1 | + 9.8%
       16  | 18.3 | 20.1 | + 9.5%
       64  | 18.3 | 26.3 | +43.8%
      128  | 20.4 | 26.3 | +28.6%
      256  | 20.0 | 26.4 | +32.1%
     1024  | 26.6 | 26.7 | + 0.2%
     8192  |541.3 | 33.5 | -93.8%

    The microbenchmark scans indexes in order, if the pattern is more
    random xa starts to win at 512 devices already. But that's a lot
    of devices, in practice.

obviously not a very realistic load, but I wanted to mention that,
in case you were planning to do similar testing. Having
wasted few hours yesterday reimplementing tls fixes Sabrina
already posted makes me hyper-vigilant now about people 
repeating work ;)

> This patch removes unpleasant code to something more readable.
> 
> As a bonus, /proc/net/dev gets netdevices sorted by their ifindex.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Jakub Kicinski <kuba@...nel.org>