netdev - Re: [PATCH net-next v4 af-packet 2/2] Enhance af-packet to provide (near zero)lossless packet capture functionality.

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Date:	Fri, 19 Aug 2011 10:22:00 -0400
From:	chetan loke <loke.chetan@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v4 af-packet 2/2] Enhance af-packet to provide
 (near zero)lossless packet capture functionality.

On Fri, Aug 19, 2011 at 6:36 AM, David Miller <davem@...emloft.net> wrote:
> From: Chetan Loke <loke.chetan@...il.com>
> Date: Thu, 11 Aug 2011 22:31:00 -0400
>
>> +struct kbdq_ft_ops {
>> +     int num_ops;
>> +     void (*ft_ops[2])(void *, void *);
>> +};
> ...
>> +     struct kbdq_ft_ops kfops;
>  ...
>> +static void prb_init_ft_ops(struct kbdq_core *p1,
>> +                     union tpacket_req_u *req_u)
>> +{
>> +     p1->kfops.ft_ops[p1->kfops.num_ops++] = prb_fill_vlan_info;
>> +
>> +     if (req_u->req3.tp_feature_req_word) {
>> +             if (req_u->req3.tp_feature_req_word & TP_FT_REQ_FILL_RXHASH)
>> +                     p1->kfops.ft_ops[p1->kfops.num_ops++] = prb_fill_rxhash;
>> +             else
>> +                     p1->kfops.ft_ops[p1->kfops.num_ops++] =
>> +                     prb_clear_rxhash;
>> +     }
>> +}
>
> It is a lot cheaper to just test the flags in-line than do indirect calls.
> Indirect calls are very expensive on many cpus.
>
> In fact, since the first op (prb_fill_vlan_info) is unconditional, we eat
> the indirect call cost for absolutely no reason at all.

Oki. Will test the flags in-line in prb_run_all_ft_ops().


>
> This kfops stuff was not present in your previous changes.  And I'm
> going to tell you that if you keep adding things each revision instead
> of just fixing the specific items you've received feedback about, a
> set of changes this invasive and of this size will never get merged.
>

First, thanks(to you and others) for reviewing the huge patchset. I
did think about breaking it into multiple patches but since it
involves changes in a single file it was a little difficult. And I
didn't want to create a separate file for this.

> Please resist the urge to further tinker with the code, and just
> address the feedback we give you.
>
Sorry won't happen. These changes should have been present in v3 when
I added the feature-request word because that was the intent of that
word. It was a screw up on my part.


Chetan Loke
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