Re: [PATCH 3/3 v2] perf tool: give user better message if precise isnot supported

From: Ingo Molnar
Date: Fri Sep 14 2012 - 07:43:30 EST

* David Ahern <dsahern@xxxxxxxxx> wrote:

> On 9/13/12 11:43 PM, Ingo Molnar wrote:
> >>v2: softened message to 'may not be' supported per Robert's suggestion
> >
> > Well, either it's supported on this machine or it's not -
> > why does the text have to be so unsure about it?
> Because EOPNOTSUPP is returned for more than just precise
> mode. We cannot say with certainty that the precise attribute
> caused that errno.

Well, then that is useful information we *lost*, and that
situation needs to be improved on the ABI side: an expanded
error code present in the event structure, copied back to
user-space on errors, or so.

(Alternatively, a special event channel just to pass back
expanded error conditions.)

Computers are supposed to make life easier for humans, by
answering such "what did go wrong?" questions. Our losing of
precise error conditions is a usability bug really - and in the
perf project we are in a unique position to be able to improve
both the kernel side code and make immediate use of it on the
tooling side as well.


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