From: Jiri Pirko <jiri@resnulli.us>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: netdev@vger.kernel.org, davem@davemloft.net, jhs@mojatatu.com,
xiyou.wangcong@gmail.com, mlxsw@mellanox.com,
edumazet@google.com, daniel@iogearbox.net,
alexander.h.duyck@intel.com, willemb@google.com,
john.fastabend@gmail.com
Subject: Re: [patch net-next v3 2/2] net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath
Date: Wed, 1 Nov 2017 09:18:03 +0100 [thread overview]
Message-ID: <20171101081803.GB1977@nanopsycho.orion> (raw)
In-Reply-To: <20171101021248.624bvt5jcqr37w5e@ast-mbp>
Wed, Nov 01, 2017 at 03:12:50AM CET, alexei.starovoitov@gmail.com wrote:
>On Tue, Oct 31, 2017 at 04:12:22PM +0100, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@mellanox.com>
>>
>> In sch_handle_egress and sch_handle_ingress tp->q is used only in order
>> to update stats. So stats and filter list are the only things that are
>> needed in clsact qdisc fastpath processing. Introduce new mini_Qdisc
>> struct to hold those items. Also, introduce a helper to swap the
>> mini_Qdisc structures in case filter list head changes.
>>
>> This removes need for tp->q usage without added overhead.
>>
>> Signed-off-by: Jiri Pirko <jiri@mellanox.com>
>> ---
>> v2->v3:
>> - Using head change callback to replace miniq pointer every time tp head
>> changes. This eliminates one rcu dereference and makes the claim "without
>> added overhead" valid.
>
>you kidding, right?
>It's still two loads.
I'm not.
I replace:
one rcu_dereference_bh(dev->egress_cl_list)
by one rcu_dereference_bh(dev->miniq_egress)
one dereference cl->q
by one dereference miniq->filter_list
What do I miss?
>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 24ac908..1423cf4 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -3274,22 +3274,22 @@ EXPORT_SYMBOL(dev_loopback_xmit);
>> static struct sk_buff *
>> sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
>> {
>> - struct tcf_proto *cl = rcu_dereference_bh(dev->egress_cl_list);
>> + struct mini_Qdisc *miniq = rcu_dereference_bh(dev->miniq_egress);
>> struct tcf_result cl_res;
>>
>> - if (!cl)
>> + if (!miniq)
>> return skb;
>>
>> /* qdisc_skb_cb(skb)->pkt_len was already set by the caller. */
>> - qdisc_bstats_cpu_update(cl->q, skb);
>> + mini_qdisc_bstats_cpu_update(miniq, skb);
>>
>> - switch (tcf_classify(skb, cl, &cl_res, false)) {
>> + switch (tcf_classify(skb, miniq->filter_list, &cl_res, false)) {
>
>I don't think it's great, but I don't have any suggestions on
>how to avoid it, so I'm not objecting. Just disappointed that
>you keep adding stuff to tc and messing with sw fast path only to
>make parity with some obscure hw feature.
>If it keeps going like this we'd need to come up with some new fast
>hook for clsbpf in ingress/egress paths. We use it for
>every packet, so extra loads are not great.
>I guess they should be cache hits, but will take extra cache line.
>All of the bugs in tc logic recently are not comforting either.
>
next prev parent reply other threads:[~2017-11-01 8:18 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-31 15:12 [patch net-next v3 0/2] net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath Jiri Pirko
2017-10-31 15:12 ` [patch net-next v3 1/2] net: sched: introduce chain_head_change callback Jiri Pirko
2017-10-31 15:12 ` [patch net-next v3 2/2] net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath Jiri Pirko
2017-11-01 2:12 ` Alexei Starovoitov
2017-11-01 8:18 ` Jiri Pirko [this message]
2017-11-01 16:11 ` Alexei Starovoitov
2017-11-02 11:27 ` Jiri Pirko
2017-11-01 10:25 ` Daniel Borkmann
2017-11-01 10:30 ` Jiri Pirko
2017-11-03 1:24 ` [patch net-next v3 0/2] " David Miller
2017-11-03 6:57 ` Jiri Pirko
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