Re: [PATCH 3/5] tcp, flow: Replace per-connection in_epoll flag with epollfd in flow_common

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 7 Oct 2025 14:07:39 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> On 07/10/2025 11:51, Stefano Brivio wrote:
> > On Tue, 7 Oct 2025 17:07:37 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> >> On Fri, Oct 03, 2025 at 05:27:15PM +0200, Laurent Vivier wrote:  
> >>> The in_epoll boolean flag in tcp_tap_conn and tcp_splice_conn only tracked
> >>> whether a connection was registered with epoll, not which epoll instance.
> >>> This limited flexibility for future multi-epoll support.
> >>>
> >>> Replace the boolean with an epollfd field in flow_common that serves dual
> >>> purpose: zero indicates not registered (replacing in_epoll=false), non-zero  
> >>
> >> Don't use 0, since that's a valid fd.
> >>  
> >>> stores the actual epoll fd (replacing in_epoll=true).  
> >>
> >> I am a bit nervous about adding 31-bits to every flow, since I think
> >> we're fairly close to a cacheline threshold.  
> > 
> > I mentioned to Laurent in our weekly call on Monday the same concern
> > and that I would look into it (first step, I would say, actually check
> > with pahole?) and, if it's an issue, into possible tricks to avoid
> > making the struct too big. I still plan to do this.
> 
> I think it already doesn't fit in a cacheline:
> 
> struct flow_common {
>          enum flow_state            state:8;              /*     0: 0  4 */
>          enum flow_type             type:8;               /*     0: 8  4 */
> 
>          /* Bitfield combined with next fields */
> 
>          uint8_t                    pif[2];               /*     2     2 */
>          struct flowside            side[2];              /*     4    72 */
> 
>          /* size: 76, cachelines: 2, members: 4 */
>          /* last cacheline: 12 bytes */
> };

Look at union flow, as we have an array of those, not flow_common. It's
two cachelines at the moment, we shouldn't go beyond that, and keeping
some margin would be nice as TCP might need some more bits eventually.

-- 
Stefano