Re: [PATCH net-next] ipv6: elide flowlabel check if no exclusive leases exist

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Sun,  7 Jul 2019 05:34:45 -0400

> From: Willem de Bruijn <willemb@google.com>
> 
> Processes can request ipv6 flowlabels with cmsg IPV6_FLOWINFO.
> If not set, by default an autogenerated flowlabel is selected.
> 
> Explicit flowlabels require a control operation per label plus a
> datapath check on every connection (every datagram if unconnected).
> This is particularly expensive on unconnected sockets multiplexing
> many flows, such as QUIC.
> 
> In the common case, where no lease is exclusive, the check can be
> safely elided, as both lease request and check trivially succeed.
> Indeed, autoflowlabel does the same even with exclusive leases.
> 
> Elide the check if no process has requested an exclusive lease.
> 
> fl6_sock_lookup previously returns either a reference to a lease or
> NULL to denote failure. Modify to return a real error and update
> all callers. On return NULL, they can use the label and will elide
> the atomic_dec in fl6_sock_release.
> 
> This is an optimization. Robust applications still have to revert to
> requesting leases if the fast path fails due to an exclusive lease.
> 
> Changes RFC->v1:
>   - use static_key_false_deferred to rate limit jump label operations
>     - call static_key_deferred_flush to stop timers on exit
>   - move decrement out of RCU context
>   - defer optimization also if opt data is associated with a lease
>   - updated all fp6_sock_lookup callers, not just udp
> 
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Looks good, applied, thanks Willem.