netdev - Re: [PATCH net-next v2] ipv4: fib: Replay events when registering FIB notifier

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Message-ID: <1478009999.7065.334.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Tue, 01 Nov 2016 07:19:59 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jiri@...lanox.com,
        mlxsw@...lanox.com, roopa@...ulusnetworks.com,
        dsa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        andy@...yhouse.net, vivien.didelot@...oirfairelinux.com,
        andrew@...n.ch, f.fainelli@...il.com, alexander.h.duyck@...el.com,
        kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
        kaber@...sh.net, Ido Schimmel <idosch@...lanox.com>
Subject: Re: [PATCH net-next v2] ipv4: fib: Replay events when registering
 FIB notifier

On Tue, 2016-11-01 at 00:57 +0200, Ido Schimmel wrote:
> On Mon, Oct 31, 2016 at 02:24:06PM -0700, Eric Dumazet wrote:

> > How well will this work for large FIB tables ?
> > 
> > Holding rtnl while sending thousands of skb will prevent consumers to
> > make progress ?
> 
> Can you please clarify what do you mean by "while sending thousands of
> skb"? This patch doesn't generate notifications to user space, but
> instead invokes notification routines inside the kernel. I probably
> misunderstood you.
> 
> Are you suggesting this be done using RCU instead? Well, there are a
> couple of reasons why I took RTNL here:
> 

No, I do not believe RCU is wanted here, in control path where we might
sleep anyway.

> 1) The FIB notification chain is blocking, so listeners are expected to
> be able to sleep. This isn't possible if we use RCU. Note that this
> chain is mainly useful for drivers that reflect the FIB table into a
> capable device and hardware operations usually involve sleeping.
> 
> 2) The insertion of a single route is done with RTNL held. I didn't want
> to differentiate between both cases. This property is really useful for
> listeners, as they don't need to worry about locking in writer-side.
> Access to data structs is serialized by RTNL.

My concern was that for large iterations, you might hold RTNL and/or
current cpu for hundred of ms or even seconds...