net: Check skb->rxhash in gro_receive

On Fri, 2014-01-10 at 08:27 -0800, Tom Herbert wrote:
> On Thu, Jan 9, 2014 at 9:38 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > On Thu, 2014-01-09 at 20:54 -0800, Tom Herbert wrote:
> >> When initializing a gro_list for a packet, first check the rxhash of
> >> the incoming skb against that of the skb's in the list. This should be
> >> a very strong inidicator of whether the flow is going to be matched,
> >> and potentially allows a lot of other checks to be short circuited.
> >>
> >
> > Hmm... this idea was discussed in the past. I used it when attempting to
> > use a hash table instead of a gro_list last year.
> >
> > Unfortunately this added lot of cycles when rxhash is not provided by
> > hardware, and my tests found it was not a win.
> >
> > Remember : in most cases, gro_list contains one flow, so this test does
> > nothing special but adds overhead.
> 
> I don't understand what your basis is that gro_list in most cases
> contains one flow, but assuming that were true, maybe we should make
> the it only contain one flow eliminating the complexity of multiple
> flows (same_flow logic is convoluted and layers of encapsulation is
> not going to simplify things).
> 

The complexity of GRO is the aggregation itself. You wont avoid it.

> If we are doing RPS or RFS, rxhash will be computed anyway, so the
> case your optimizing is pretty narrow: no RPS, no RFS, no hardware
> hash, and a single flow in gro_list. Nevertheless, if this is really
> an important concern, we can make the check directly against
> skb->rxhash so to be opportunistic and avoid the possibility of
> computation.


We'll compute rxhash once per GRO packet, containing up to 40 MSS
packets.

Thats a big difference.

If your patch was doing this, I would have no complain.

(No new conditional branch, and skb->rxhash, if provided by the NIC,
can give a hint.)




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On Fri, Jan 10, 2014 at 9:45 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Fri, 2014-01-10 at 08:27 -0800, Tom Herbert wrote:
>> On Thu, Jan 9, 2014 at 9:38 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> > On Thu, 2014-01-09 at 20:54 -0800, Tom Herbert wrote:
>> >> When initializing a gro_list for a packet, first check the rxhash of
>> >> the incoming skb against that of the skb's in the list. This should be
>> >> a very strong inidicator of whether the flow is going to be matched,
>> >> and potentially allows a lot of other checks to be short circuited.
>> >>
>> >
>> > Hmm... this idea was discussed in the past. I used it when attempting to
>> > use a hash table instead of a gro_list last year.
>> >
>> > Unfortunately this added lot of cycles when rxhash is not provided by
>> > hardware, and my tests found it was not a win.
>> >
>> > Remember : in most cases, gro_list contains one flow, so this test does
>> > nothing special but adds overhead.
>>
>> I don't understand what your basis is that gro_list in most cases
>> contains one flow, but assuming that were true, maybe we should make
>> the it only contain one flow eliminating the complexity of multiple
>> flows (same_flow logic is convoluted and layers of encapsulation is
>> not going to simplify things).
>>
>
> The complexity of GRO is the aggregation itself. You wont avoid it.
>
>> If we are doing RPS or RFS, rxhash will be computed anyway, so the
>> case your optimizing is pretty narrow: no RPS, no RFS, no hardware
>> hash, and a single flow in gro_list. Nevertheless, if this is really
>> an important concern, we can make the check directly against
>> skb->rxhash so to be opportunistic and avoid the possibility of
>> computation.
>
>
> We'll compute rxhash once per GRO packet, containing up to 40 MSS
> packets.
>
> Thats a big difference.
>
The objective is to compute the rxhash at most once per packet.

> If your patch was doing this, I would have no complain.
>
> (No new conditional branch, and skb->rxhash, if provided by the NIC,
> can give a hint.)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ce01847793c0..c9f7a26d7ce7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3798,7 +3798,8 @@ static void gro_list_prepare(struct napi_struct *napi, struct sk_buff *skb)
>         for (p = napi->gro_list; p; p = p->next) {
>                 unsigned long diffs;
>
> -               diffs = (unsigned long)p->dev ^ (unsigned long)skb->dev;
> +               diffs = p->rxhash ^ skb->rxhash;
> +               diffs |= (unsigned long)p->dev ^ (unsigned long)skb->dev;
>                 diffs |= p->vlan_tci ^ skb->vlan_tci;
>                 if (maclen == ETH_HLEN)
>                         diffs |= compare_ether_header(skb_mac_header(p),
>
As I said in the commit log, the skb->rxhash should be a very strong
indicator of that flows will match (maybe >99% ?), so putting that
first potentially short circuits a lot of work even in just this
function.

>
>
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On Fri, 2014-01-10 at 10:15 -0800, Tom Herbert wrote:

> The objective is to compute the rxhash at most once per packet.

Once per GRO packet ( up to 45 MSS) or once per incoming frame ( 1 MSS
) ?

Your patch computes rxhash for every incoming frame.

> 
> > If your patch was doing this, I would have no complain.
> >
> > (No new conditional branch, and skb->rxhash, if provided by the NIC,
> > can give a hint.)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index ce01847793c0..c9f7a26d7ce7 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -3798,7 +3798,8 @@ static void gro_list_prepare(struct napi_struct *napi, struct sk_buff *skb)
> >         for (p = napi->gro_list; p; p = p->next) {
> >                 unsigned long diffs;
> >
> > -               diffs = (unsigned long)p->dev ^ (unsigned long)skb->dev;
> > +               diffs = p->rxhash ^ skb->rxhash;
> > +               diffs |= (unsigned long)p->dev ^ (unsigned long)skb->dev;
> >                 diffs |= p->vlan_tci ^ skb->vlan_tci;
> >                 if (maclen == ETH_HLEN)
> >                         diffs |= compare_ether_header(skb_mac_header(p),
> >
> As I said in the commit log, the skb->rxhash should be a very strong
> indicator of that flows will match (maybe >99% ?), so putting that
> first potentially short circuits a lot of work even in just this
> function.

Are you speaking of your "goto skip;" ?

compare_ether_header() is done with 10 instructions on x86_64

There is no point trying to avoid it.

Really, 99% of the time gro_list contains zero or one single slot, I
have hard data saying so.

If you want to optimize the case where list is fully populated (because
of yet another synthetic benchmark you use), you really need to build a
temporary list so that all layers do not even have to check
NAPI_GRO_CB(p)->same_flow

Each gro handler would remove non matching flow from this temp list.

-> when we finally reach tcp_gro_receive(), list would contain a single
element.



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> Really, 99% of the time gro_list contains zero or one single slot, I
> have hard data saying so.
>
Please provide the data.

> If you want to optimize the case where list is fully populated (because
> of yet another synthetic benchmark you use), you really need to build a
> temporary list so that all layers do not even have to check
> NAPI_GRO_CB(p)->same_flow
>
Well if you prefer, you can think of the "synthetic benchmark" as
emulating an obvious DOS attack by pumping MSS sized TCP segments with
random ports to a server. The stack needs to be resilient to such
things, an O(n*m) algorithm in the data path is a red flag.

> Each gro handler would remove non matching flow from this temp list.
>
> -> when we finally reach tcp_gro_receive(), list would contain a single
> element.
>
>
>
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