git: cb0dd7e122b8 - main - tcp: reduce memory footprint when listing tcp hostcache

[Richard Scheffenegger]

The branch main has been updated by rscheff:

URL: https://cgit.FreeBSD.org/src/commit/?id=cb0dd7e122b8936ad61a141e65ef8ef874bfebe5

commit cb0dd7e122b8936ad61a141e65ef8ef874bfebe5
Author:     Richard Scheffenegger <rscheff at FreeBSD.org>
AuthorDate: 2021-03-28 21:12:03 +0000
Commit:     Richard Scheffenegger <rscheff at FreeBSD.org>
CommitDate: 2021-03-28 21:50:23 +0000

    tcp: reduce memory footprint when listing tcp hostcache
    
    In tcp_hostcache_list, the sbuf used would need a large (~2MB)
    blocking allocation of memory (M_WAITOK), when listing a
    full hostcache. This may stall the requestor for an indeterminate
    time.
    
    A further optimization is to return the expected userspace
    buffersize right away, rather than preparing the output of
    each current entry of the hostcase, provided by: @tuexen.
    
    This makes use of the ready-made functions of sbuf to work
    with sysctl, and repeatedly drain the much smaller buffer.
    
    PR: 254333
    MFC after: 2 weeks
    Reviewed By: #transport, tuexen
    Sponsored by: NetApp, Inc.
    Differential Revision: https://reviews.freebsd.org/D29471
---
 sys/netinet/tcp_hostcache.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/sys/netinet/tcp_hostcache.c b/sys/netinet/tcp_hostcache.c
index de999bcfda22..5fe905d9abf5 100644
--- a/sys/netinet/tcp_hostcache.c
+++ b/sys/netinet/tcp_hostcache.c
@@ -628,7 +628,7 @@ sysctl_tcp_hc_list(SYSCTL_HANDLER_ARGS)
 {
 	const int linesize = 128;
 	struct sbuf sb;
-	int i, error;
+	int i, error, len;
 	struct hc_metrics *hc_entry;
 	char ip4buf[INET_ADDRSTRLEN];
 #ifdef INET6
@@ -638,11 +638,22 @@ sysctl_tcp_hc_list(SYSCTL_HANDLER_ARGS)
 	if (jailed_without_vnet(curthread->td_ucred) != 0)
 		return (EPERM);
 
-	sbuf_new(&sb, NULL, linesize * (V_tcp_hostcache.cache_count + 1),
-		SBUF_INCLUDENUL);
+	/* Optimize Buffer length query by sbin/sysctl */
+	if (req->oldptr == NULL) {
+		len = (V_tcp_hostcache.cache_count + 1) * linesize;
+		return (SYSCTL_OUT(req, NULL, len));
+	}
+
+	error = sysctl_wire_old_buffer(req, 0);
+	if (error != 0) {
+		return(error);
+	}
+
+	/* Use a buffer for 16 lines */
+	sbuf_new_for_sysctl(&sb, NULL, 16 * linesize, req);
 
 	sbuf_printf(&sb,
-	        "\nIP address        MTU  SSTRESH      RTT   RTTVAR "
+		"\nIP address        MTU  SSTRESH      RTT   RTTVAR "
 		"    CWND SENDPIPE RECVPIPE HITS  UPD  EXP\n");
 
 #define msec(u) (((u) + 500) / 1000)
@@ -677,8 +688,6 @@ sysctl_tcp_hc_list(SYSCTL_HANDLER_ARGS)
 	}
 #undef msec
 	error = sbuf_finish(&sb);
-	if (error == 0)
-		error = SYSCTL_OUT(req, sbuf_data(&sb), sbuf_len(&sb));
 	sbuf_delete(&sb);
 	return(error);
 }