[linux-cifs-client] Re: What is GetXid/FreeXid supposed to actually do?

[Jeff Layton]

On Sun, 24 May 2009 20:36:25 -0500
Steve French <smfrench at gmail.com> wrote:

> On Sun, May 24, 2009 at 4:03 PM, Jeff Layton <jlayton at redhat.com> wrote:
> > On Sun, 24 May 2009 15:38:22 -0500
> > Steve French <smfrench at gmail.com> wrote:
> >
> >> On Sun, May 24, 2009 at 2:31 PM, Christoph Hellwig <hch at infradead.org> wrote:
> >> > On Sun, May 24, 2009 at 03:21:07PM -0400, Jeff Layton wrote:
> >> >> Any objection to just replacing this xid value with the pid or tgid
> >> >> instead? Does this xid value give you anything that that doesn't?
> >> >
> >> > Yeah, from all that I can see there is always at most one xid in
> >> > use per thread.  So just replacing it with a tgid should work easily.
> >> > Even better would be replacing all that ad-hoc debugging with the
> >> > ftrace event tracer.
> >>
> >> That may work (using the tgid) but there are various cases where it was
> >> helpful to distinguish log entries this way (rather than simply using
> >> timestamps,
> >> especially since dmesg is lossy so you don't always see the function entry
> >> and exits) when you have multiple requests in a series from one or more threads
> >> request from thread1, request from thread 2, request 2 from thread1,
> >> request 2 from thread 2 etc.
> >>
> >
> > dmesg is never lossy...maybe you mean syslog?
> 
> Yes - in particular running dbench with logging on you tend to lose
> a lot, and when I add in debugging messages (using the xid)
> it helps sort them.
> 
> > Anyway, there are never multiple requests in flight from single threads
> > in cifs. In all cases, a thread transmits a request and then goes to
> > sleep until the reply comes in.
> 
> There are a couple of rare cases, but usually this (xid) is to untangle
> the case of lots of requests in the log.
> 

That might be the case if it ever got printed with every message, but
it looks like it almost never does.

A more useful scheme would be to make the tgid get printed whenever a
cFYI/cERROR message is printed. With that we could get rid of all of
this GetXid/FreeXid infrastructure.

> > AFAICT too, this xid only ever shows up in one cERROR message:
> >
> >    cERROR(1, ("Frame too large received.  Length: %d  Xid: %d",
> >                receive_len, xid));
> >
> > ...that just doesn't seem like it's worth the cost.
> 
> In the past I also have used it (xid) a lot in debugging through /proc/fs/cifs/
> 

I'm not sure I follow this. This xid is only showing up in one or two
error messages as far as I can tell. How does it get used
under /proc/fs/cifs?

> >
> >> Using the ftrace event tracer may be good enough for some of this
> >> (except the stats gathering, counters) - any pointers to its
> >> use in NFS?
> >>
> >
> > I know Steve D. has been working on tracepoints for nfs, but I haven't
> > given them a hard look as of yet.
> 
> Some guys (OLS speakers) from India were talking about fs tracepoints last
> year at OLS but I got the feeling that not much progress had been made.
> 

I think a lot of progress has been made in the last year on this. At
this point, it does look like ftrace is the way of the future. LWN has
a good article on this if you have access:

    http://lwn.net/Articles/322666/

...we'd want to add static tracepoints:

    http://lwn.net/Articles/330402/

...though I think the interface may have changed a little since that
article was written.

-- 
Jeff Layton <jlayton at redhat.com>