Re: [PATCH v2 5/5] vfio: ccw: add traceponits for interesting error paths

Cornelia Huck
2018-04-30T11:51:53Z

On Sat, 28 Apr 2018 13:50:23 +0800
Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com> wrote:

> * Cornelia Huck <cohuck@redhat.com> [2018-04-27 12:13:53 +0200]:
>
> > On Mon, 23 Apr 2018 13:01:13 +0200
> > Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com> wrote:
> >
> > typo in subject: s/traceponits/tracepoints/
> >
> > > From: Halil Pasic <pasic@linux.vnet.ibm.com>
> > >
> > > Add some tracepoints so we can inspect what is not working as is should.
> > >
> > > Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> > > Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
> > > ---
> > > drivers/s390/cio/Makefile | 1 +
> > > drivers/s390/cio/vfio_ccw_fsm.c | 16 +++++++-
> > > drivers/s390/cio/vfio_ccw_trace.h | 77 +++++++++++++++++++++++++++++++++++++++
> > > 3 files changed, 93 insertions(+), 1 deletion(-)
> > > create mode 100644 drivers/s390/cio/vfio_ccw_trace.h
> >
> >
> > > @@ -135,6 +142,8 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> > > goto err_out;
> > >
> > > io_region->ret_code = cp_prefetch(&private->cp);
> > > + trace_vfio_ccw_cp_prefetch(get_schid(private),
> > > + io_region->ret_code);
> > > if (io_region->ret_code) {
> > > cp_free(&private->cp);
> > > goto err_out;
> > > @@ -142,11 +151,13 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> > >
> > > /* Start channel program and wait for I/O interrupt. */
> > > io_region->ret_code = fsm_io_helper(private);
> > > + trace_vfio_ccw_fsm_io_helper(get_schid(private),
> > > + io_region->ret_code);
> > > if (io_region->ret_code) {
> > > cp_free(&private->cp);
> > > goto err_out;
> > > }
> > > - return;
> > > + goto out;
> > > } else if (scsw->cmd.fctl & SCSW_FCTL_HALT_FUNC) {
> > > /* XXX: Handle halt. */
> > > io_region->ret_code = -EOPNOTSUPP;
> > > @@ -159,6 +170,9 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> > >
> > > err_out:
> > > private->state = VFIO_CCW_STATE_IDLE;
> > > +out:
> > > + trace_vfio_ccw_io_fctl(scsw->cmd.fctl, get_schid(private),
> > > + io_region->ret_code);
> > > }
> > >
> > > /*
> >
> > I really don't want to bikeshed, especially as some tracepoints are
> > better than no tracepoints, but...
> >
> > We now trace fctl/schid/ret_code unconditionally (good).
> >
> > We trace the outcome of cp_prefetch() and fsm_io_helper()
> > unconditionally. We don't, however, trace all things that may go wrong.
> > We have the tracepoint at the end, but it cannot tell us where the
> > error came from. Should we have tracepoints in every place (in this
> > function) that may generate an error? Only if there is an actual error?
> > Are the two enough for common debug scenarios?
> Trace actual error sounds like a better idea than trace unconditionally
> of these two functions.
> These two are not enough for common debug scenarios. For example, we
> cann't tell if a -EOPNOTSUPP is a orb->tm.b problem, or error code
> returned by cp_init().
>
> Idea to improve:
> 1. Trace actual error.
> 2. Define a trace event and add error trace for cp_init().

Hm. Going from what I have done in the past when doing printk debugging:

- stick in a message that is always hit, with some information about
parameters, if it makes sense
- stick in a message "foo happened!" in the error branches
- or, alternatively, trace the called functions

So tracing on failure only might be more useful? Have all failure paths
under a common knob to turn on/off?

> > Opinions? We can just go ahead with this and improve things later
> > on, I guess.
> >
> I think it's also fine to do this - better something than nothing. We
> could at least have a code base to be improved to make everybody
> happier in future.

Maybe keep the patch as it is now, except trace the errors only
(keeping the fctl trace point)?

Halil, as you wrote the patch (and I presume you found it helpful):
What is your opinion?