From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTP id EDE985A027A for ; Sat, 2 Dec 2023 05:35:04 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701491703; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=cDF8rfzIXgx3d1nYY6d11AAn4PQcseL+2nkOaQ3jPsc=; b=R3c39auFa6VhuGALosriFq/snLIejPG+Es+ClIcJ5Cwhbtedcmn5LON+fG5zfE49d79vjz TisXzO9g5PLifE/KsVRpVanCTiLPdNo77nv7cd/TDtyuLwXRlKIQZOE17cnj4qr4Ioa3+/ aljpv7J6xgL4ldXXcBEpDBDt4Aj7sPk= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-625-QvoXOLIuPVG993AumWdMZA-1; Fri, 01 Dec 2023 23:35:02 -0500 X-MC-Unique: QvoXOLIuPVG993AumWdMZA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id AD5101C04323; Sat, 2 Dec 2023 04:35:01 +0000 (UTC) Received: from elisabeth (unknown [10.39.208.34]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CFADB36E2; Sat, 2 Dec 2023 04:35:00 +0000 (UTC) Date: Sat, 2 Dec 2023 05:34:58 +0100 From: Stefano Brivio To: David Gibson Subject: Re: [PATCH v3 16/16] tcp: Don't defer hash table removal Message-ID: <20231202053458.2390d4fb@elisabeth> In-Reply-To: References: <20231130020222.4056647-1-david@gibson.dropbear.id.au> <20231130020222.4056647-17-david@gibson.dropbear.id.au> <20231130134532.7c5b959e@elisabeth> Organization: Red Hat MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID-Hash: JDFHGR7HT4WIAKNBXDRPXAX74U434YPM X-Message-ID-Hash: JDFHGR7HT4WIAKNBXDRPXAX74U434YPM X-MailFrom: sbrivio@redhat.com 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: 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. -- Stefano