Re: [RFD] perf syscall error handling

[Arnaldo Carvalho de Melo]

Em Mon, Nov 10, 2014 at 01:24:47PM +0100, Ingo Molnar escreveu:
> * Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > Em Mon, Nov 10, 2014 at 11:27:25AM +0100, Ingo Molnar escreveu:
> > > * Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > > > Em Mon, Nov 03, 2014 at 05:50:19PM +0100, Peter Zijlstra escreveu:
> > > > > OK, so how about we do both, the offset+mask for the tools 
> > > > > and the string for the humans?

It looks like machines don't have problems with strings 8-)

> > > > Yeah, tooling tries to provide the best it can with the 
> > > > offset+mask, and if doesn't manage to do anything smart with 
> > > > it, just show the string and hope that helps the user to figure 
> > > > out what is happening.

> > > Almost: tooling should generally always consider the string as 
> > > well, for the (not so uncommon) case where there can be multiple 
> > > problems with the same field.

> > > Really, I think the string will give the most bang for the buck, 
> > > because it's really simple and straightforward on the kernel side 
> > > (so that we have a good chance of achieving full coverage 
> > > relatively quickly), and later on we could still complicate it 
> > > all with offset+mask if there's really a need.

> > > So lets start with an error string...

> > I don't have a problem with the order of introduction of new 
> > error reporting mechanisms, or at least I can't think of one 
> > right now.

> > So if we introduce strings now then tools/perf/ will trow them 
> > to the user when it still don't have fallbacks or any other UI 
> > indication of such an error.

> > I wonder tho if we have any previous experience on some other 
> > project (or even in the kernel?) and how userspace ended up 
> > using it, if just presenting those strings to the user or if 
> > trying to parse it, etc, anybody?

> I'm not aware of any such efforts in the Linux space - subsystems 
> with administrative interfaces generally just tend to printk() a 
> reason - that's obviously suboptimal in several ways.
 
> Programmatic use in user-spaec is very simple - go with my 
> initial example, tooling can either just display the error string 
> and bail out, or do:
 
>   if (unlikely(error)) {
> 	if (!strcmp(attr->error_str, "x86/bts: BTS not supported by this CPU architecture")) {
> 		fprintf(stderr, "x86/BTS: No hardware support falling back to branch sampling\n");
> 		activate_x86_bts_fallback_code();
> 		goto out;
> 	}
> 	if (!strcmp(attr->error_str, "x86/lbr: LBR not supported by this CPU architecture"))
> 		goto out_err;
>   }
 
> or it may do any number of other things, such as convert it to 
> its internal error code. Note that the error messages should have 
> some minimal structure (the 'x86/bts:' and 'x86/lbr' prefixes) to 
> organize things nicely and to make string clashes less likely.

Right, focus on the string format: Can we just have this two level
thing, first part separated by a slash, followed by colon, to identify
the origin of the message, and then a message, that can have further,
unspecified at this time, parser tokens as the need arises?
 
> as this is a slowpath the performance of strcmp() doesn't matter, 
> and in any case it's hardware accelerated or optimized well on 
> most platforms.

- Arnaldo
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