sflow: Fix sFlow sampling structure.
According to Neil McKee, in an email archived at
http://openvswitch.org/pipermail/dev_openvswitch.org/2010-January/000934.html:
The containment rule is that a given sflow-datasource (sampler or
poller) should be scoped within only one sflow-agent (or
sub-agent). So the issue arrises when you have two
switches/datapaths defined on the same host being managed with
the same IP address: each switch is a separate sub-agent, so they
can run independently (e.g. with their own sequence numbers) but
they can't both claim to speak for the same sflow-datasource.
Specifically, they can't both represent the <ifindex>:0
data-source. This containment rule is necessary so that the
sFlow collector can scale and combine the results accurately.
One option would be to stick with the <ifindex>:0 data-source but
elevate it to be global across all bridges, with a global
sample_pool and a global sflow_agent. Not tempting. Better to
go the other way and allow each interface to have it's own
sampler, just as it already has it's own poller. The ifIndex
numbers are globally unique across all switches/datapaths on the
host, so the containment is now clean. Datasource <ifindex>:5
might be on one switch, whille <ifindex>:7 can be on another.
Other benefits are that 1) you can support the option of
overriding the default sampling-rate on an interface-by-interface
basis, and 2) this is how most sFlow implementations are coded,
so there will be no surprises or interoperability issues with any
sFlow collectors out there.
This commit implements the approach suggested by Neil.
This commit uses an atomic_t to represent the sampling pool. This is
because we do want access to it to be atomic, but we expect that it will
"mostly" be accessed from a single CPU at a time. Perhaps this is a bad
assumption; we can always switch to another form of synchronization later.
CC: Neil McKee <neil.mckee@inmon.com>
projects / openvswitch / commit