From: Jamal Hadi Salim <jhs@mojatatu•com>
To: John Fastabend <john.fastabend@gmail•com>
Cc: John Fastabend <john.r.fastabend@intel•com>,
Cong Wang <xiyou.wangcong@gmail•com>,
Eric Dumazet <eric.dumazet@gmail•com>,
Linux Kernel Network Developers <netdev@vger•kernel.org>,
"David S. Miller" <davem@davemloft•net>
Subject: Re: [RFC Patch net-next] net_sched: make classifying lockless on ingress
Date: Sun, 22 Dec 2013 11:01:06 -0500 [thread overview]
Message-ID: <52B70CC2.9080805@mojatatu.com> (raw)
In-Reply-To: <52B61FA3.9050904@gmail.com>
On 12/21/13 18:09, John Fastabend wrote:
>
> I solved this by making them per CPU and synchronizing when I hit
> an operation that required sync'ing them. Going forward if folks
> have the time to write SMP aware qdisc's that work with eventually
> consistent counters that would be great.
>
I think what you describe is reasonable as well. Need to weigh pro/con
of both.
> You could make this fully generic by having a classifer to match
> the cpu id and then forwarding the skb to a qdisc based on the
> cpu_id.
>
Indeed. More a "generic stateless metadata" classifier which may
look at more than just cpu id to make the systolic decision.
Probably at pre-enqueu that you had.
I would assume the amount of config update on such a table would be
very very minimal - so RCU would do well.
If someone wants more deeper lookup, then a tc classifier would make
more sense. But by default the generic stateless classifier maybe
sufficient.
> Then per-netdev-ingress-per-cpu is really just a configured policy.
> If we wanted to make it the default configuration that would be
> fine.
>
Perhaps thats just defensive talk on my part when people say "qdiscs
are slow". No - netdevs are slow. Rephrase: Netdevs are shared across
CPUs, you MUST lock. Locks create cache misses etc.
As an example, I dont know why something like RPS could not have
benefited from the rich classifier-action if someone wanted it to.
So there may still be need for per-netdev-ingress-per-cpu.
cheers,
jamal
next prev parent reply other threads:[~2013-12-22 16:01 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-20 23:28 [RFC Patch net-next] net_sched: make classifying lockless on ingress Cong Wang
2013-12-20 23:49 ` Eric Dumazet
2013-12-20 23:57 ` Cong Wang
2013-12-21 0:08 ` Eric Dumazet
2013-12-21 0:24 ` Cong Wang
2013-12-21 2:32 ` John Fastabend
2013-12-21 22:11 ` Jamal Hadi Salim
2013-12-21 23:09 ` John Fastabend
2013-12-22 16:01 ` Jamal Hadi Salim [this message]
2013-12-24 0:56 ` Cong Wang
2013-12-24 6:08 ` John Fastabend
2013-12-26 12:02 ` Jamal Hadi Salim
2013-12-21 1:09 ` John Fastabend
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