Re: Trying to reduce NFSv4 timeouts to a few seconds on an established connection
From: Jeff Layton <jlayton@kernel.org>
Date: 2023-01-27 13:33:19
On Thu, 2023-01-26 at 22:08 +0000, Andrew Klaassen wrote:
quoted
From: Andrew Klaassen <redacted> Sent: Thursday, January 26, 2023 10:32 AMquoted
From: Andrew Klaassen <redacted> Sent: Monday, January 23, 2023 11:31 AM Hello, There's a specific NFSv4 mount on a specific machine which we'd like to timeout and return an error after a few seconds if the server goes away. I've confirmed the following on two different kernels, 4.18.0- 348.12.2.el8_5.x86_64 and 6.1.7-200.fc37.x86_64. I've been able to get both autofs and the mount command to cooperate, so that the mount attempt fails after an arbitrary number of seconds. This mount command, for example, will fail after 6 seconds, as expected based on the timeo=20,retrans=2,retry=0 options: $ time sudo mount -t nfs4 -o rw,relatime,sync,vers=4.2,rsize=131072,wsize=131072,namlen=255,acr egmi n=0,acregmax=0,acdirmin=0,acdirmax=0,soft,noac,proto=tcp,timeo=20,ret raquoted
n s=2,retry=0,sec=sys thor04:/mnt/thorfs04 /mnt/thor04 mount.nfs4: Connection timed out real 0m6.084s user 0m0.007s sys 0m0.015s However, if the share is already mounted and the server goes away, the timeout is always 2 minutes plus the time I expect based on timeo and retrans. In this case, 2 minutes and 6 seconds: $ time ls /mnt/thor04 ls: cannot access '/mnt/thor04': Connection timed out real 2m6.025s user 0m0.003s sys 0m0.000s Watching the outgoing packets in the second case, the pattern is always the same: - 0.2 seconds between the first two, then doubling each time until the two minute mark is exceeded (so the last NFS packet, which is always the 11th packet, is sent around 1:45 after the first). - Then some generic packets that start exactly-ish on the two minute mark, 1 second between the first two, then doubling each time. (By this time the NFS command has given up.) 11:10:21.898305 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834889483 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:22.105189 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834889690 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:22.313290 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834889898 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:22.721269 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834890306 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:23.569192 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834891154 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:25.233212 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834892818 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:28.497282 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834896082 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:35.025219 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834902610 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:10:48.337201 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834915922 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:11:14.449303 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834942034 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:12:08.721251 IP 10.30.13.2.916 > 10.31.3.13.2049: Flags [P.], seq 14452:14652, ack 18561, win 501, options [nop,nop,TS val 834996306 ecr 1589769203], length 200: NFS request xid 3614904256 196 getattr fh 0,2/53 11:12:22.545394 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835010130 ecr 0,nop,wscale 7], length 0 11:12:23.570199 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835011155 ecr 0,nop,wscale 7], length 0 11:12:25.617284 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835013202 ecr 0,nop,wscale 7], length 0 11:12:29.649219 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835017234 ecr 0,nop,wscale 7], length 0 11:12:37.905274 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835025490 ecr 0,nop,wscale 7], length 0 11:12:54.289212 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835041874 ecr 0,nop,wscale 7], length 0 11:13:26.545304 IP 10.30.13.2.942 > 10.31.3.13.2049: Flags [S], seq 1375256951, win 64240, options [mss 1460,sackOK,TS val 835074130 ecr 0,nop,wscale 7], length 0 I tried changing tcp_retries2 as suggested in another thread from this list: # echo 3 > /proc/sys/net/ipv4/tcp_retries2 ...but it made no difference on either kernel. The 2 minute timeout also doesn't seem to match with what I'd calculate from the initial value of tcp_retries2, which should give a much higher timeout. The only clue I've been able to find is in the retry=n entry in the NFS manpage: " For TCP the default is 3 minutes, but system TCP connection timeouts will sometimes limit the timeout of each retransmission to around 2minutes."quoted
What I'm not able to make sense of: - The retry option says that it applies to mount operations, not read/write operations. However, in this case I'm seeing the 2 minute delay on read/write operations but *not* mount operations. - A couple of hours of searching didn't lead me to any kernel settings that would result in a 2 minute timeout. Does anyone have any clues about a) what's happening and b) how to get our desired behaviour of being able to control both mount and read/write timeouts down to a few seconds? Thanks.I thought that changing TCP_RTO_MAX in include/net/tcp.h from 120 to something smaller and recompiling the kernel would change the 2 minute timeout, but it had no effect. I'm going to keep poking through the kernel code to see if there's a knob I can turn to change the 2 minute timeout, so that I can at least understand where it's coming from. Any hints as to where I should be looking?I believe I've made some progress with this today: - Calls to rpc_create() from fs/nfs/client.c are sending an rpc_timeout struct with their args. - rpc_create() does *not* pass the timeout on to xprt_create_transport(), which then can't pass it on to xs_setup_tcp(). - xs_setup_tcp(), having no timeout passed to it, uses xs_tcp_default_timeout instead. - changing xs_tcp_default_timeout changes the "ls" timeout behaviour I described above. In theory all of this means that the timeout simply needs to be passed through and used instead of xs_tcp_default_timeout. I'm going to give this a try tomorrow.
That's a great root-cause analysis. The interlocking timeouts involved with NFS and its sockets can be really difficult to unwind. Is there a way to automate this testcase? That might be nice to have in xfstests or the nfstest suite.
quoted hunk ↗ jump to hunk
Here's what I'm going to try first; I'm no C programmer, though, so any advice or corrections you might have would be appreciated. Thanks. Andrewdiff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c index 0b0b9f1eed46..1350c1f489f7 100644 --- a/net/sunrpc/clnt.c +++ b/net/sunrpc/clnt.c@@ -532,6 +532,7 @@ struct rpc_clnt *rpc_create(struct rpc_create_args*args) .addrlen = args->addrsize, .servername = args->servername, .bc_xprt = args->bc_xprt, + .timeout = args->timeout, }; char servername[48]; struct rpc_clnt *clnt;diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c index aaa5b2741b79..adc79d94b59e 100644 --- a/net/sunrpc/xprtsock.c +++ b/net/sunrpc/xprtsock.c@@ -3003,7 +3003,7 @@ static struct rpc_xprt *xs_setup_tcp(structxprt_create *args) xprt->idle_timeout = XS_IDLE_DISC_TO; xprt->ops = &xs_tcp_ops; - xprt->timeout = &xs_tcp_default_timeout; + xprt->timeout = args->timeout; xprt->max_reconnect_timeout = xprt->timeout->to_maxval; xprt->connect_timeout = xprt->timeout->to_initval *
Looks like you're probably on the right track. You're missing a few things: You'll need to add a "timeout" field to struct xprt_create in include/linux/sunrpc/xprt.h, and there may be some other places that either need to set the timeout in that structure, or do something with that field when it's set. Once you have something that fixes your reproducer, go ahead and post it and we can help you work through whatever changes need to me made to make it work. Nice work! -- Jeff Layton [off-list ref]