netdev - Re: [patch net-next 06/34] net: core: use dev->ingress

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Date:   Sun, 15 Oct 2017 01:18:54 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Pirko <jiri@...nulli.us>
CC:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, mlxsw@...lanox.com, andrew@...n.ch,
        vivien.didelot@...oirfairelinux.com, f.fainelli@...il.com,
        michael.chan@...adcom.com, ganeshgr@...lsio.com,
        jeffrey.t.kirsher@...el.com, saeedm@...lanox.com,
        matanb@...lanox.com, leonro@...lanox.com, idosch@...lanox.com,
        jakub.kicinski@...ronome.com, ast@...nel.org,
        simon.horman@...ronome.com, pieter.jansenvanvuuren@...ronome.com,
        john.hurley@...ronome.com, edumazet@...gle.com, dsahern@...il.com,
        alexander.h.duyck@...el.com, john.fastabend@...il.com,
        willemb@...gle.com
Subject: Re: [patch net-next 06/34] net: core: use dev->ingress_queue instead
 of tp->q

On 10/13/2017 08:30 AM, Jiri Pirko wrote:
> Thu, Oct 12, 2017 at 11:45:43PM CEST, daniel@...earbox.net wrote:
>> On 10/12/2017 07:17 PM, Jiri Pirko wrote:
>>> From: Jiri Pirko <jiri@...lanox.com>
>>>
>>> In sch_handle_egress and sch_handle_ingress, don't use tp->q and use
>>> dev->ingress_queue which stores the same pointer instead.
>>>
>>> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
>>> ---
>>>    net/core/dev.c | 21 +++++++++++++++------
>>>    1 file changed, 15 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>> index fcddccb..cb9e5e5 100644
>>> --- a/net/core/dev.c
>>> +++ b/net/core/dev.c
>>> @@ -3273,14 +3273,18 @@ EXPORT_SYMBOL(dev_loopback_xmit);
>>>    static struct sk_buff *
>>>    sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
>>>    {
>>> +	struct netdev_queue *netdev_queue =
>>> +				rcu_dereference_bh(dev->ingress_queue);
>>>    	struct tcf_proto *cl = rcu_dereference_bh(dev->egress_cl_list);
>>>    	struct tcf_result cl_res;
>>> +	struct Qdisc *q;
>>>
>>> -	if (!cl)
>>> +	if (!cl || !netdev_queue)
>>>    		return skb;
>>> +	q = netdev_queue->qdisc;
>>
>> NAK, no additional overhead in the software fast-path of
>> sch_handle_{ingress,egress}() like this. There are users out there
>> that use tc in software only, so performance is critical here.
>
> Okay, how else do you suggest I can avoid the need to use tp->q?
> I was thinking about storing q directly to net_device, which would safe
> one dereference, resulting in the same amount as current cl->q.

Sorry for late reply, mostly off for few days. netdev struct has different
cachelines which are hot on rx and tx (see also the location of the two
lists, ingress_cl_list and egress_cl_list), if you add only one qdisc
pointer there, then you'd at least penalize one of the two w/ potential
cache miss. Can we leave it in cl?