netdev - Re: [PATCH net-next v2] gtp: add notification mechanism

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Date:   Thu, 27 Aug 2020 00:36:24 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Harald Welte <laforge@...monks.org>
Cc:     netdev@...r.kernel.org, osmocom-net-gprs@...ts.osmocom.org,
        Gabriel Ganne <gabriel.ganne@...nd.com>, kuba@...nel.org,
        davem@...emloft.net, pablo@...filter.org
Subject: Re: [PATCH net-next v2] gtp: add notification mechanism

Le 26/08/2020 à 20:52, Harald Welte a écrit :
> Hi Nicolas,
> 
> On Wed, Aug 26, 2020 at 09:47:54AM +0200, Nicolas Dichtel wrote:
>>> Sending (unsolicited) notifications about all of those seems quite heavyweight to me.
>>
>> There is no 'unsolicited' notifications with this patch. Notifications are sent
>> only if a userspace application has subscribed to the gtp mcast group.
>> ip routes or conntrack entries are notified in the same way and there could a
>> lot of them also (more than 100k conntrack entries for example).
> 
> Ok, thanks for reminding me of that.  However, even if those events are
> not sent/multicasted, it still looks like the proposed patch is
> unconditionally allocating a netlink message and filling it with
> information about the PDP.  That alone looks like adding significant
> overhead to every user - even the majority of current use cases where
> nobody is listening/subscribing to that multicast group.
I don't think that this is a significant overhead. This is added in the control
path. When a PDP context is added, the rtnl lock is took, this is another
magnitude of overhead than a kmalloc().

> 
> Wouldn't it make sense to only allocate + fill those messages if we
> actually knew a subscriber existed?
In fact, this is actually how the netlink framework works.