net/sched: conditional notification of events for cls and act

This is an optimization we have been leveraging on P4TC but we believe
it will benefit rtnl users in general.

It's common to allocate an skb, build a notification message and then
broadcast an event. In the absence of any user space listeners, these
resources (cpu and memory operations) are wasted. In cases where the subsystem
is lockless (such as in tc-flower) this waste is more prominent. For the
scenarios where the rtnl_lock is held it is not as prominent.

The idea is simple. Build and send the notification iif:
   - The user requests via NLM_F_ECHO or
   - Someone is listening to the rtnl group (tc mon)

On a simple test with tc-flower adding 1M entries, using just a single core,
there's already a noticeable difference in the cycles spent in tc_new_tfilter
with this patchset.

before:
   - 43.68% tc_new_tfilter
      + 31.73% fl_change
      + 6.35% tfilter_notify
      + 1.62% nlmsg_notify
        0.66% __tcf_qdisc_find.part.0
        0.64% __tcf_chain_get
        0.54% fl_get
      + 0.53% tcf_proto_lookup_ops

after:
   - 39.20% tc_new_tfilter
      + 34.58% fl_change
        0.69% __tcf_qdisc_find.part.0
        0.67% __tcf_chain_get
      + 0.61% tcf_proto_lookup_ops

Note, the above test is using iproute2:tc which execs a shell.
We expect people using netlink directly to observe even greater
reductions.

The qdisc side needs some refactoring of the notification routines to fit in
this new model, so they will be sent in a later patchset.

v3->v4:
- Collected Simon review tags
- Addressed Simon comments
- Added 2 more patches

v2->v3:
- Collected Jiri review tags
- Fixed commit messages to refer to rtnl_notify_needed

v1->v2:
- Addressed Jakub comments

Jamal Hadi Salim (1):
  rtnl: add helper to check if rtnl group has listeners

Pedro Tammela (5):
  rtnl: add helper to send if skb is not null
  net/sched: act_api: don't open code max()
  net/sched: act_api: conditional notification of events
  net/sched: cls_api: remove 'unicast' argument from delete notification
  net/sched: cls_api: conditional notification of events

Victor Nogueira (1):
  rtnl: add helper to check if a notification is needed

 include/linux/rtnetlink.h |  29 +++++++++++
 net/sched/act_api.c       | 107 +++++++++++++++++++++++++++-----------
 net/sched/cls_api.c       |  38 ++++++++------
 3 files changed, 128 insertions(+), 46 deletions(-)

-- 
2.40.1