Re: [PATCH v3 3/3] perf probe: Show better error message when failed to find variable

[He Kuang]

On 2015/5/11 17:50, Ingo Molnar wrote:
> * He Kuang <hekuang@xxxxxxxxxx> wrote:
>
> > Hi, Ingo
> >
> > On 2015/5/11 17:30, Ingo Molnar wrote:
> > > * He Kuang <hekuang@xxxxxxxxxx> wrote:
> > >
> > > > Indicate to check variable location range in error message when we got
> > > > failed to find the variable.
> > > >
> > > > Before this patch:
> > > >
> > > >    $ perf probe --add 'generic_perform_write+118 bytes'
> > > >    Failed to find the location of bytes at this address.
> > > >     Perhaps, it has been optimized out.
> > > >      Error: Failed to add events.
> > > >
> > > > After this patch:
> > > >    $ perf probe --add 'generic_perform_write+118 bytes'
> > > >    Failed to find the location of bytes at this address.
> > > What does this sentence mean? I thought 'address' means 'location of
> > > bytes'. So the address identifies the location and obviously we know
> > > that. So this message wants to say something else.
> > 'generic_perform_write' is a function name, while 'bytes' is a local
> > variable in this function. Maybe the variable I chose make you confused.
> >
> > This maybe clear:
> >    Failed to find the location of 'bytes' at this address.
> Yeah, absolutely! This highlights the importance of putting
> user-supplied symbols into quotes and such.
>
> Maybe even write:
>
>      Failed to find the location of the 'bytes' variable at this address.
>
> Another question: what does 'generic_perform_write+118' mean here? Why
> the offset?

This is for setting a kprobe at the location has an offset of 118
bytes to the entry of the function 'generic_perform_write'.

In the previous patch: https://lkml.org/lkml/2015/5/11/170,
we show a valid byte offset range of variables in scope, 118 is
an invalid address which can be read from the result, this offset
is just for showing the error message.

> Thanks,
>
> 	Ingo
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