Subject: Re: [RFC] Could we avoid touching dst->refcount in some cases ?

From: Andi Kleen <[email-masked]>
Date: Mon, 24 Nov 2008 10:42:42 +0100

Eric Dumazet <[email-masked]> writes:

> tbench has hard time incrementing decrementing the route cache refcount
> shared by all communications on localhost.

iirc there was a patch some time ago to use per CPU loopback devices to 
avoid this, but it was considered too much a benchmark hack.
As core counts increase it might stop being that though.

>
> On real world, we also have this problem on RTP servers sending many UDP
> frames to mediagateways, especially big ones handling thousand of streams.
>
> Given that route entries are using RCU, we probably can avoid incrementing
> their refcount in case of connected sockets ?

Normally they can be hold over sleeps or queuing of skbs too, and RCU
doesn't handle that. To make it handle that you would need to define a
custom RCU period designed for this case, but this would be probably
tricky and fragile: especially I'm not sure even if you had a "any
packet queued" RCU method it be guaranteed to always finish 
because there is no fixed upper livetime of a packet.

The other issue is that on preemptible kernels you would need to 
disable preemption all the time such a routing entry is hold, which
could be potentially quite long.

-Andi

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