On Sun, 2018-12-02 at 12:07 -0500, Chuck Lever wrote:
> Test report.
>
> I applied this on top of the three v4 patches from Friday.
>
> With TCP, I no longer see the soft IRQ warnings or bdev leaks.
> However, at some point during my test (sec=krb5p,vers=3) the
> mount stops responding and the client becomes idle. ^C leaves
> the iozone processes in D state.
Does /proc/*/stack show any tasks with any interesting stack behaviour?
>
> With RDMA, I can see a few issues:
> - A lot of -74 and server disconnects. I suspect this is due
> to GSS sequence window overruns
> - xprt_connect puts the calling task to sleep if the credit
> limit has been reached, and that can result in a deadlock
> - The reconnect logic appears to busy-wait until something
> else (eg, an RTO) forces a reconnect
> - A connect or disconnect wake-up that occurs while an RPC
> reply is being handled (ie, is still on xprt->pending)
> results in that RPC being retried unnecessarily.
>
> I'm not sure how to address that last one. Sometimes RDMA has
> to invalidate MRs, and that involves a wait/context swith which
> opens the race window significantly.
In principle the latter issue is supposed to be handled by the connect
cookies. Are the updates perhaps being ordered incorrectly w.r.t. the
xprt->pending wakeup? If so, then that could cause races.
I'd expect the cookie would need to be updated before the call to xprt-
>pending both on connect and disconnect.
>
> > On Dec 2, 2018, at 9:42 AM, Trond Myklebust <trondmy@xxxxxxxxx>
> > wrote:
> >
> > If an asynchronous connection attempt completes while another task
> > is
> > in xprt_connect(), then the call to rpc_sleep_on() could end up
> > racing with the call to xprt_wake_pending_tasks().
> > So add a second test of the connection state after we've put the
> > task to sleep and set the XPRT_CONNECTING flag, when we know that
> > there
> > can be no asynchronous connection attempts still in progress.
> >
> > Fixes: 0b9e79431377d ("SUNRPC: Move the test for XPRT_CONNECTING
> > into...")
> > Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
> > ---
> > net/sunrpc/xprt.c | 11 +++++++++--
> > 1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> > index 122c91c28e7c..ce927002862a 100644
> > --- a/net/sunrpc/xprt.c
> > +++ b/net/sunrpc/xprt.c
> > @@ -826,8 +826,15 @@ void xprt_connect(struct rpc_task *task)
> > return;
> > if (xprt_test_and_set_connecting(xprt))
> > return;
> > - xprt->stat.connect_start = jiffies;
> > - xprt->ops->connect(xprt, task);
> > + /* Race breaker */
> > + if (!xprt_connected(xprt)) {
> > + xprt->stat.connect_start = jiffies;
> > + xprt->ops->connect(xprt, task);
> > + } else {
> > + xprt_clear_connecting(xprt);
> > + task->tk_status = 0;
> > + rpc_wake_up_queued_task(&xprt->pending, task);
> > + }
> > }
> > xprt_release_write(xprt, task);
> > }
> > --
> > 2.19.2
> >
>
> --
> Chuck Lever
>
>
>
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx
