tcp: Allocate t_tcpreq_info on demand

[gallatin]

Summary

When TCP_REQUEST_TRK is enabled, the tcb grows by 600 bytes
to accommodate the t_tcpreq_info[MAX_TCP_TRK_REQ] array.
Even when the option is enabled, not every connection is using
this feature. So let's allocate it on-demand, and save 600
bytes in the common case.

[glebius]

Given this is purely a RACK feature, I would suggest to move it from generic struct tcpcb to struct tcp_rack.

[glebius]

Two options: 1) embed all of this in struct tcp_rack:

#ifdef TCP_REQUEST_TRK
        /* Response tracking addons. */
        uint8_t t_tcpreq_req;   /* Request count */
        uint8_t t_tcpreq_open;  /* Number of open range requests */
        uint8_t t_tcpreq_closed;        /* Number of closed range requests */
        uint32_t tcp_hybrid_start;      /* Num of times we started hybrid pacing */
        uint32_t tcp_hybrid_stop;       /* Num of times we stopped hybrid pacing */
        uint32_t tcp_hybrid_error;      /* Num of times we failed to start hybrid pacing */
        struct tcp_sendfile_track *t_tcpreq_info;
#endif

or 2) create a new structure that holds all of the above, lazily allocate it and have a pointer from struct tcp_rack to it.

[gallatin]

In D56484#1292880, @glebius wrote:

Two options: 1) embed all of this in struct tcp_rack:

#ifdef TCP_REQUEST_TRK
        /* Response tracking addons. */
        uint8_t t_tcpreq_req;   /* Request count */
        uint8_t t_tcpreq_open;  /* Number of open range requests */
        uint8_t t_tcpreq_closed;        /* Number of closed range requests */
        uint32_t tcp_hybrid_start;      /* Num of times we started hybrid pacing */
        uint32_t tcp_hybrid_stop;       /* Num of times we stopped hybrid pacing */
        uint32_t tcp_hybrid_error;      /* Num of times we failed to start hybrid pacing */
        struct tcp_sendfile_track *t_tcpreq_info;
#endif

or 2) create a new structure that holds all of the above, lazily allocate it and have a pointer from struct tcp_rack to it.

The same could be said of TCP_ACCOUNTING as well.

In any case, I'd prefer to at least start with this. What you suggest would be much more invasive and require more testing. This is a simple change that delivers 90% of the benefit in terms of shrinking the tcb without a lot of churn or testing required.

[glebius]

I'm not blocking your change. Just suggesting.

[rrs]

I like this first cut simple approach Gleb !!