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

[Halil Pasic]

On 04/30/2018 01:51 PM, Cornelia Huck wrote:
> On Sat, 28 Apr 2018 13:50:23 +0800
> Dong Jia Shi <bjsdjshi@xxxxxxxxxxxxxxxxxx> wrote:
>
> > * Cornelia Huck <cohuck@xxxxxxxxxx> [2018-04-27 12:13:53 +0200]:
> >
> > > On Mon, 23 Apr 2018 13:01:13 +0200
> > > Dong Jia Shi <bjsdjshi@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > typo in subject: s/traceponits/tracepoints/
> > >    
> > > > From: Halil Pasic <pasic@xxxxxxxxxxxxxxxxxx>
> > > >
> > > > Add some tracepoints so we can inspect what is not working as is should.
> > > >
> > > > Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Dong Jia Shi <bjsdjshi@xxxxxxxxxxxxxxxxxx>
> > > > ---
> > > >   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)?

What do you mean by this sentence. Get rid of vfio_ccw_io_fctl or get
rid of vfio_ccw_cp_prefetch and vfio_ccw_fsm_io_helper, or get don't
get rid of any, but make some conditional (!errno)?

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

I'm in favor of this patch (as previously stated here
https://patchwork.kernel.org/patch/10298305/). And regarding the
questions under discussion I'm mostly fine either way.

I think the naming of this fctl thing is a bit cryptic,
but if we don't see this as ABI I'm fine with it -- can be improved.
What would be a better name? I was thinking along the lines accept_request.
(Bad error code would mean that the request did not get accepted. Good
code does not mean the requested function was performed successfully.)

Also I think vfio_ccw_io_fctl with no zero error code would make sense
as dev_warn. If I were an admin looking into a problem I would very much
appreciate seeing something in the messages log (and not having to enable
tracing first). This point seems to be a good one for high level 'request gone
bad' kind of report. Opinions?

Regards,
Halil