netdev - Re: [PATCH RFC net-next] net: cache the __dev_alloc

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Date: Tue, 7 May 2024 21:24:36 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: William Tu <witu@...dia.com>
Cc: <netdev@...r.kernel.org>, <jiri@...dia.com>, <bodong@...dia.com>,
 <kuba@...nel.org>
Subject: Re: [PATCH RFC net-next] net: cache the __dev_alloc_name()

On Mon, 6 May 2024 20:32:07 +0000
William Tu <witu@...dia.com> wrote:

> When a system has around 1000 netdevs, adding the 1001st device becomes
> very slow. The devlink command to create an SF
>   $ devlink port add pci/0000:03:00.0 flavour pcisf \
>     pfnum 0 sfnum 1001
> takes around 5 seconds, and Linux perf and flamegraph show 19% of time
> spent on __dev_alloc_name() [1].
> 
> The reason is that devlink first requests for next available "eth%d".
> And __dev_alloc_name will scan all existing netdev to match on "ethN",
> set N to a 'inuse' bitmap, and find/return next available number,
> in our case eth0.
> 
> And later on based on udev rule, we renamed it from eth0 to
> "en3f0pf0sf1001" and with altname below
>   14: en3f0pf0sf1001: <BROADCAST,MULTICAST,UP,LOWER_UP> ...
>       altname enp3s0f0npf0sf1001
> 
> So eth0 is actually never being used, but as we have 1k "en3f0pf0sfN"
> devices + 1k altnames, the __dev_alloc_name spends lots of time goint
> through all existing netdev and try to build the 'inuse' bitmap of
> pattern 'eth%d'. And the bitmap barely has any bit set, and it rescanes
> every time.
> 
> I want to see if it makes sense to save/cache the result, or is there
> any way to not go through the 'eth%d' pattern search. The RFC patch
> adds name_pat (name pattern) hlist and saves the 'inuse' bitmap. It saves
> pattens, ex: "eth%d", "veth%d", with the bitmap, and lookup before
> scanning all existing netdevs.
> 
> Note: code is working just for quick performance benchmark, and still
> missing lots of stuff. Using hlist seems to overkill, as I think
> we only have few patterns
> $ git grep alloc_netdev drivers/ net/ | grep %d
> 
> 1. https://github.com/williamtu/net-next/issues/1
> 
> Signed-off-by: William Tu <witu@...dia.com>

Actual patch is bit of a mess, with commented out code, leftover printks,
random whitespace changes. Please fix that.

The issue is that bitmap gets to be large and adds bloat to embedded devices.

Perhaps you could either force devlink to use the same device each time (eth0)
if it is going to be renamed anyway.