From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from gandalf.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id DBA365A027A for ; Mon, 4 Dec 2023 01:45:59 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202312; t=1701650753; bh=DwuOhMbiOwyEO5LB9ZbTfD+1uYlVJ3hBryqFJ0xw+Xk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=nd/zFzJefIew/TAdGHso1vmVsSqC07+st8padsrd1cfe6oh7EZGJR+7Pl3TGnw3fX eXKuOd80zTedO0LuthGHikuAeQhh9udQPA7AgB0CyDwJv4jHqeSpNbWlYoOaEEzxew 4nGvf8tW6MLw30n6pkBzkLxLGeFciNsPSzCP3KqhYP1BtflqpA+wGJiOXoAJwZV7zt G3cYsisD4wjNfjLOSUHS4GsqUAwT6Gy82lCj0WJRQPYU1KdzEbjs2i7RB7kDDWPDA6 ZJ72eLvUAjtMHTfBH/rVAMT8+GSnum0anEt3EGqJ0S9e3kgojT04RBF9ikLbRwRwZI 91MlqSPOX8Ggw== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4Sk4hx5Ynwz4xh3; Mon, 4 Dec 2023 11:45:53 +1100 (AEDT) Date: Mon, 4 Dec 2023 11:43:11 +1100 From: David Gibson To: Stefano Brivio Subject: Re: [PATCH v3 16/16] tcp: Don't defer hash table removal Message-ID: References: <20231130020222.4056647-1-david@gibson.dropbear.id.au> <20231130020222.4056647-17-david@gibson.dropbear.id.au> <20231130134532.7c5b959e@elisabeth> <20231202053458.2390d4fb@elisabeth> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="PgPHotEcU/ZCItnv" Content-Disposition: inline In-Reply-To: <20231202053458.2390d4fb@elisabeth> Message-ID-Hash: WGG4IJO4M6E4ODA6PGHD55WVJHM5FG45 X-Message-ID-Hash: WGG4IJO4M6E4ODA6PGHD55WVJHM5FG45 X-MailFrom: dgibson@gandalf.ozlabs.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: --PgPHotEcU/ZCItnv Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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: >=20 > > 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: > > > =20 > > > > When a TCP connection is closed, we mark it by setting events to CL= OSED, > > > > then some time later we do final cleanups: closing sockets, removin= g from > > > > the hash table and so forth. > > > >=20 > > > > This does mean that when making a hash lookup we need to exclude any > > > > apparent matches that are CLOSED, since they represent a stale conn= ection. > > > > This can happen in practice if one connection closes and a new one = with the > > > > same endpoints is started shortly afterward. > > > >=20 > > > > Checking for CLOSED is quite specific to TCP however, and won't wor= k 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. > > > >=20 > > > > Signed-off-by: David Gibson > > > > --- > > > > tcp.c | 10 +++++++--- > > > > 1 file changed, 7 insertions(+), 3 deletions(-) > > > >=20 > > > > 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, s= truct tcp_tap_conn *conn, > > > > tcp_timer_ctl(c, conn); > > > > } > > > > =20 > > > > +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 s= tate > > > > * @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 =3D=3D -1 ? "CLOSED" : tcp_event_str[num]); > > > > =20 > > > > - if ((event =3D=3D TAP_FIN_RCVD) && !(conn->events & SOCK_FIN_RCVD= )) > > > > + if (event =3D=3D CLOSED) > > > > + tcp_hash_remove(c, conn); > > > > + else if ((event =3D=3D 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_ta= p_conn *conn, > > > > const union inany_addr *faddr, > > > > in_port_t eport, in_port_t fport) > > > > { > > > > - if (conn->events !=3D CLOSED && inany_equals(&conn->faddr, faddr)= && > > > > + if (inany_equals(&conn->faddr, faddr) && > > > > conn->eport =3D=3D eport && conn->fport =3D=3D fport) > > > > return 1; > > > > =20 > > > > @@ -1308,7 +1313,6 @@ static void tcp_conn_destroy(struct ctx *c, u= nion flow *flow) > > > > if (conn->timer !=3D -1) > > > > close(conn->timer); > > > > =20 > > > > - tcp_hash_remove(c, conn); > > > > flow_table_compact(c, flow); =20 > > >=20 > > > 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. > > >=20 > > > 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. =20 > >=20 > > 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. > >=20 > > > 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. =20 > >=20 > > Right.. to process a FIN and the next SYN at once, I guess? >=20 > That's one example, yes, but in any case it was an optimisation for... >=20 > > 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. >=20 > 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. --=20 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 --PgPHotEcU/ZCItnv Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmVtII4ACgkQzQJF27ox 2Gcz1xAAn45PUCTmauQeiUzrw7xy5D6jvpubGQdL9cmtmJVIwzi4sK/cisfH2IHa yq5UqiIqHcKg3aAMrJB68UEfEKxybXzgFbjhowrrpQwLb7JyO2LtVXiWfkYHPum2 Sz10+u4LFzebcBU9Naz6C+i5pEACpyKhRxL9HyKZMSu4DTZZtXz/IUepkwrWmHgg znge+//O5AkHqMi1EkgDs9QJ+lMCc/Kj1FWK5Rk6cb/wgr9Jw3Sdai0PwhOdlvhA b57yLcjeziDj34rLNGZc5Z41++2l9nj4gtNiiGTh0Yt+vFuZNjWI+uMoykboixKZ v7whhT5inkyJeMTNU+wWipg5PWUTqL3yZhotXqJzTtCpgiu6no2Q+Ln7nxabwAI7 hubh/I7xMPUi+4AO8YbCs4x4ESUrplZSWOPbXJ4pCGQDQ5wXxJ1P2ezXiuogZnp+ QKVxjmtcudru0yGxg+QL+H9yj+zS/3mvYg90i+U3ZUcu5TAXbgaYsDtgpQsKuM2a TtLqMNl8ExqKpXgV0iMrgmF2ADh4urGSR9yV+mnIKFp78ZPCCnfHwvQp28h6EFXi PiEJPazrPD4Ry0eWHAFGjJmp/LupeTUKOGAgg9N+21PHMWWUE+3CukMUDt21qfIg kSdWoktH5eCLB6yCBOLdQd7NiAUfNMPq+or2ep0sfgGpJGGK2EE= =q88Y -----END PGP SIGNATURE----- --PgPHotEcU/ZCItnv--