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From:   Olga Kornievskaia <olga.kornievskaia@gmail.com>
Date:   Thu, 9 Jul 2020 11:43:23 -0400
Message-ID: <CAN-5tyGk3aU-DRhWACMD8-NMtdfX4ANUcR3xAjjEySf-GbbA6w@mail.gmail.com>
Subject: Re: [PATCH 1/1] SUNRPC dont update timeout value on connection reset
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     "anna.schumaker@netapp.com" <anna.schumaker@netapp.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
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On Thu, Jul 9, 2020 at 8:08 AM Trond Myklebust <trondmy@hammerspace.com> wrote:
>
> Hi Olga
>
> On Wed, 2020-07-08 at 17:05 -0400, Olga Kornievskaia wrote:
> > Current behaviour: every time a v3 operation is re-sent to the server
> > we update (double) the timeout. There is no distinction between
> > whether
> > or not the previous timer had expired before the re-sent happened.
> >
> > Here's the scenario:
> > 1. Client sends a v3 operation
> > 2. Server RST-s the connection (prior to the timeout) (eg.,
> > connection
> > is immediately reset)
> > 3. Client re-sends a v3 operation but the timeout is now 120sec.
> >
> > As a result, an application sees 2mins pause before a retry in case
> > server again does not reply.
> >
> > Instead, this patch proposes to keep track off when the minor timeout
> > should happen and if it didn't, then don't update the new timeout.
> >
> > Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
> > ---
> >  include/linux/sunrpc/xprt.h |  1 +
> >  net/sunrpc/xprt.c           | 11 +++++++++++
> >  2 files changed, 12 insertions(+)
> >
> > diff --git a/include/linux/sunrpc/xprt.h
> > b/include/linux/sunrpc/xprt.h
> > index e64bd82..a603d48 100644
> > --- a/include/linux/sunrpc/xprt.h
> > +++ b/include/linux/sunrpc/xprt.h
> > @@ -101,6 +101,7 @@ struct rpc_rqst {
> >                                                        * used in the
> > softirq.
> >                                                        */
> >       unsigned long           rq_majortimeo;  /* major timeout
> > alarm */
> > +     unsigned long           rq_minortimeo;  /* minor timeout
> > alarm */
> >       unsigned long           rq_timeout;     /* Current timeout
> > value */
> >       ktime_t                 rq_rtt;         /* round-trip time */
> >       unsigned int            rq_retries;     /* # of retries */
> > diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> > index d5cc5db..c0ce232 100644
> > --- a/net/sunrpc/xprt.c
> > +++ b/net/sunrpc/xprt.c
> > @@ -607,6 +607,11 @@ static void xprt_reset_majortimeo(struct
> > rpc_rqst *req)
> >       req->rq_majortimeo += xprt_calc_majortimeo(req);
> >  }
> >
> > +static void xprt_reset_minortimeo(struct rpc_rqst *req)
> > +{
> > +     req->rq_minortimeo = jiffies + req->rq_timeout;
> > +}
> > +
> >  static void xprt_init_majortimeo(struct rpc_task *task, struct
> > rpc_rqst *req)
> >  {
> >       unsigned long time_init;
> > @@ -618,6 +623,7 @@ static void xprt_init_majortimeo(struct rpc_task
> > *task, struct rpc_rqst *req)
> >               time_init = xprt_abs_ktime_to_jiffies(task->tk_start);
> >       req->rq_timeout = task->tk_client->cl_timeout->to_initval;
> >       req->rq_majortimeo = time_init + xprt_calc_majortimeo(req);
> > +     req->rq_minortimeo = time_init + req->rq_timeout;
> >  }
> >
> >  /**
> > @@ -631,6 +637,10 @@ int xprt_adjust_timeout(struct rpc_rqst *req)
> >       const struct rpc_timeout *to = req->rq_task->tk_client-
> > >cl_timeout;
> >       int status = 0;
> >
> > +     if (time_before(jiffies, req->rq_minortimeo)) {
> > +             xprt_reset_minortimeo(req);
> > +             return status;
>
> Shouldn't this case be just returning without updating the timeout?
> After all, this is the case where nothing has expired yet.

I think we perhaps should readjust the minor timeout every here but I
can't figure out what the desired behaviour should be. When should we
consider it's appropriate to double the timer. Consider the following:

time1: v3 op sent
time1+50s: server RSTs
We check that it's not yet the minor timeout (time1+60s)
time1+50s: v3 op re-sent  (say we don't reset the minor timeout to be
current time+60s)
time1+60s: server RSTs
Client will resend the op but now it's past the initial minor timeout
so the timeout will be doubled. Is that what we really want? Maybe it
is.
Say now the server RSTs the connection again (shortly after or in less
than 60s), since we are not updating the minor timeout value, then the
client will again modify the timeout before resending. Is that Ok?

That's why my reasoning was that at every re-evaluation of the timeout
value, we have the minor timeout set for current time+60s and we get
an RST within it then we don't modify the timeout value.


>
> > +     }
> >       if (time_before(jiffies, req->rq_majortimeo)) {
> >               if (to->to_exponential)
> >                       req->rq_timeout <<= 1;
> > @@ -638,6 +648,7 @@ int xprt_adjust_timeout(struct rpc_rqst *req)
> >                       req->rq_timeout += to->to_increment;
> >               if (to->to_maxval && req->rq_timeout >= to->to_maxval)
> >                       req->rq_timeout = to->to_maxval;
> > +             xprt_reset_minortimeo(req);
>
> ...and then perhaps this can just be moved out of the time_before()
> condition, since it looks to me as if we also want to reset req-
> >rq_minortimeo when a major timeout occurs.
>
> >               req->rq_retries++;
> >       } else {
> >               req->rq_timeout = to->to_initval;
>
>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>
>