netdev - Re: [RFC PATCH net-next] net: sched: flower: refactor reoffload for concurrent access

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Message-ID: <vbf8sw8h8k6.fsf@mellanox.com>
Date:   Wed, 17 Apr 2019 17:01:36 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>
CC:     Vlad Buslov <vladbu@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [RFC PATCH net-next] net: sched: flower: refactor reoffload for
 concurrent access


On Wed 17 Apr 2019 at 19:34, Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
> On Wed, 17 Apr 2019 07:29:36 +0000, Vlad Buslov wrote:
>> On Wed 17 Apr 2019 at 00:49, Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
>> > On Tue, 16 Apr 2019 17:20:47 +0300, Vlad Buslov wrote:
>> >> @@ -1551,6 +1558,10 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
>> >>  		goto errout_mask;
>> >>
>> >>  	if (!tc_skip_hw(fnew->flags)) {
>> >> +		spin_lock(&tp->lock);
>> >> +		list_add(&fnew->hw_list, &head->hw_filters);
>> >> +		spin_unlock(&tp->lock);
>> >> +
>> >>  		err = fl_hw_replace_filter(tp, fnew, rtnl_held, extack);
>> >>  		if (err)
>> >>  			goto errout_ht;
>> >
>> > Duplicated deletes should be fine, but I'm not sure same is true for
>> > adds.  Won't seeing an add with the same cookie twice confuse drivers?
>> >
>> > There's also the minor issue of offloaded count being off in that
>> > case :)
>>
>> Hmmm, okay. Rejecting duplicate cookies should be a trivial change to
>> drivers though. Do you see any faults with this approach in general?
>
> Trivial or not it adds up, the stack should make driver authors' job as
> easy as possible.  The simplest thing to do would be to add a mutex

Agree. However, all driver flower offload implementations already have
all necessary functionality to lookup flow by cookie because they need
it to implement flow deletion.

> around the HW calls.  But that obviously doesn't work for you, cause
> you want multiple outstanding requests to the FW for a single tp,
> right?

Right.

>
> How about a RW lock, that would take R on normal add/replace/del paths
> and W on replays?  That should scale, no?

Yes. But I would prefer to avoid adding another sleeping lock on
rule update path of every filter (including non-offloaded use cases when
reoffload is not used at all).

Jiri, what approach would you prefer?