netdev - Re: [PATCH iproute2] ip link: Do not call ll_name_to

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Date:   Thu, 17 May 2018 18:17:12 -0600
From:   David Ahern <dsahern@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH iproute2] ip link: Do not call ll_name_to_index when
 creating a new link

On 5/17/18 4:36 PM, Stephen Hemminger wrote:
> On Thu, 17 May 2018 16:22:37 -0600
> dsahern@...nel.org wrote:
> 
>> From: David Ahern <dsahern@...il.com>
>>
>> Using iproute2 to create a bridge and add 4094 vlans to it can take from
>> 2 to 3 *minutes*. The reason is the extraneous call to ll_name_to_index.
>> ll_name_to_index results in an ioctl(SIOCGIFINDEX) call which in turn
>> invokes dev_load. If the index does not exist, which it won't when
>> creating a new link, dev_load calls modprobe twice -- once for
>> netdev-NAME and again for NAME. This is unnecessary overhead for each
>> link create.
>>
>> When ip link is invoked for a new device, there is no reason to
>> call ll_name_to_index for the new device. With this patch, creating
>> a bridge and adding 4094 vlans takes less than 3 *seconds*.
>>
>> Signed-off-by: David Ahern <dsahern@...il.com>
> 
> Yes this looks like a real problem.
> Isn't the cache supposed to reduce this?
> 
> Don't like to make lots of special case flags.
> 

The device does not exist, so it won't be in any cache. ll_name_to_index
already checks it though before calling if_nametoindex.