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

7 Oct 2025


      On 07/10/2025 11:51, Stefano Brivio wrote:
...
On Tue, 7 Oct 2025 17:07:37 +1100
David Gibson  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 */
};

Laurent