On Sat, Dec 02, 2023 at 05:34:58AM +0100, Stefano Brivio wrote: > On Fri, 1 Dec 2023 11:07:50 +1100 > David Gibson wrote: > > > On Thu, Nov 30, 2023 at 01:45:32PM +0100, Stefano Brivio wrote: > > > On Thu, 30 Nov 2023 13:02:22 +1100 > > > David Gibson wrote: > > > > > > > When a TCP connection is closed, we mark it by setting events to CLOSED, > > > > then some time later we do final cleanups: closing sockets, removing from > > > > the hash table and so forth. > > > > > > > > This does mean that when making a hash lookup we need to exclude any > > > > apparent matches that are CLOSED, since they represent a stale connection. > > > > This can happen in practice if one connection closes and a new one with the > > > > same endpoints is started shortly afterward. > > > > > > > > Checking for CLOSED is quite specific to TCP however, and won't work when > > > > we extend the hash table to more general flows. So, alter the code to > > > > immediately remove the connection from the hash table when CLOSED, although > > > > we still defer closing sockets and other cleanup. > > > > > > > > Signed-off-by: David Gibson > > > > --- > > > > tcp.c | 10 +++++++--- > > > > 1 file changed, 7 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/tcp.c b/tcp.c > > > > index 74d06bf..17c7cba 100644 > > > > --- a/tcp.c > > > > +++ b/tcp.c > > > > @@ -781,6 +781,9 @@ static void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn, > > > > tcp_timer_ctl(c, conn); > > > > } > > > > > > > > +static void tcp_hash_remove(const struct ctx *c, > > > > + const struct tcp_tap_conn *conn); > > > > + > > > > /** > > > > * conn_event_do() - Set and log connection events, update epoll state > > > > * @c: Execution context > > > > @@ -825,7 +828,9 @@ static void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn, > > > > flow_dbg(conn, "%s", > > > > num == -1 ? "CLOSED" : tcp_event_str[num]); > > > > > > > > - if ((event == TAP_FIN_RCVD) && !(conn->events & SOCK_FIN_RCVD)) > > > > + if (event == CLOSED) > > > > + tcp_hash_remove(c, conn); > > > > + else if ((event == TAP_FIN_RCVD) && !(conn->events & SOCK_FIN_RCVD)) > > > > conn_flag(c, conn, ACTIVE_CLOSE); > > > > else > > > > tcp_epoll_ctl(c, conn); > > > > @@ -1150,7 +1155,7 @@ static int tcp_hash_match(const struct tcp_tap_conn *conn, > > > > const union inany_addr *faddr, > > > > in_port_t eport, in_port_t fport) > > > > { > > > > - if (conn->events != CLOSED && inany_equals(&conn->faddr, faddr) && > > > > + if (inany_equals(&conn->faddr, faddr) && > > > > conn->eport == eport && conn->fport == fport) > > > > return 1; > > > > > > > > @@ -1308,7 +1313,6 @@ static void tcp_conn_destroy(struct ctx *c, union flow *flow) > > > > if (conn->timer != -1) > > > > close(conn->timer); > > > > > > > > - tcp_hash_remove(c, conn); > > > > flow_table_compact(c, flow); > > > > > > I was pretty sure, due to the way I originally implemented this, that > > > removing an entry from the hash table without compacting the table > > > afterwards, with an event possibly coming between the two, would > > > present some inconsistency while we're handling that event. > > > > > > But looking at it now, I don't see any issue with this. I just wanted > > > to raise it in case you're aware of (but didn't think about) some > > > concern in this sense. > > > > I think it's ok. The thing is that compacting the connection table > > itself is largely independent of the hash table, whose buckets are > > separately indexed. A hash remove shuffles things around in the hash > > buckets, but doesn't change where connections sit in the connection > > table. A connection table compaction changes the indices in the > > connection table, which requires updating things in the hash buckets, > > but not moving things around in the buckets - exactly the same entries > > are in every hash bucket, it's just that one of them has a new "name" > > now. > > > > > By the way, the reason why I deferred tcp_hash_remove() back then was > > > to save cycles between epoll events and get higher CRR rates, but I > > > think the effect is negligible anyway. > > > > Right.. to process a FIN and the next SYN at once, I guess? > > That's one example, yes, but in any case it was an optimisation for... > > > I figured > > this might make a difference, but probably not much. There's no > > syscall here, and batching doesn't reduce the total amount of work in > > this case. > > supposedly better data locality, with batching. But never > micro-benchmarked, and surely negligible anyway. Hmm.. except hash tables are by construction non-local, so I wouldn't really expect batching unrelated hash entries to do that. Even if the connection table entries themselves are close, which is more likely, they take a cacheline each on the most common platform, so that's not likely to win anything. In fact if anything, I'd expect better locality with the non-deferred approach - we're triggering the hash remove when we've already been working on that connection, so it should be cache hot. I guess batching does win some icache locality. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson