Messages in this thread | | | Date | Mon, 10 Nov 2014 13:24:47 +0100 | From | Ingo Molnar <> | Subject | Re: [RFD] perf syscall error handling |
| |
* Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> Em Mon, Nov 10, 2014 at 11:27:25AM +0100, Ingo Molnar escreveu: > > > > * Arnaldo Carvalho de Melo <acme@kernel.org> wrote: > > > > > Em Mon, Nov 03, 2014 at 05:50:19PM +0100, Peter Zijlstra escreveu: > > > > On Mon, Nov 03, 2014 at 02:25:48PM -0200, Arnaldo Carvalho de Melo wrote: > > > > > > > > The way that peterz suggested, i.e. returning information about which > > > > > perf_event_attr and which of the parameters was invalid/had issues could > > > > > help with fallbacking/capability querying, i.e. tooling may want to use > > > > > some features if available automagically, fallbacking to something else > > > > > when that fails. > > > > > > > > We already do that to some degree in various cases, but for some if the > > > > > only way that becomes available to disambiguate some EINVAL return is a > > > > > string, code will start having strcmps :-\ > > > > > > > OK, so how about we do both, the offset+mask for the tools > > > > and the string for the humans? > > > > > > 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.
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.
Thanks,
Ingo
| |