Re: [RFC] perf/sdt: Directly record SDT event with 'perf record'

[Arnaldo Carvalho de Melo]

Em Thu, Feb 23, 2017 at 01:43:38PM +0530, Ravi Bangoria escreveu:
> Thanks Arnaldo,
> 
> I'm working on this but it's taking bit longer time. Will post out a patch within
> few days.

Take your time and thanks for giving consideration to my observations,

Regards,

- Arnaldo
 
> Ravi
> 
> On Monday 20 February 2017 07:41 PM, Arnaldo Carvalho de Melo wrote:
> > Em Mon, Feb 20, 2017 at 04:31:50PM +0530, Ravi Bangoria escreveu:
> >> Thanks Ingo,
> >>
> >> On Monday 20 February 2017 02:12 PM, Ingo Molnar wrote:
> >>> * Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx> wrote:
> >>>
> >>>> What should be the behavior of the tool? Should it record only one
> >>>> 'sdt_libpthread:mutex_entry' which exists in uprobe_events? Or it
> >>>> should record all the SDT events from libpthread? We can choose either
> >>>> of two but both the cases are ambiguous.
> >>> They are not ambiguous really if coded right: just pick one of the outcomes and 
> >>> maybe print a warning to inform the user that something weird is going on because 
> >>> not all markers are enabled?
> >>>
> >>> As a user I'd expect 'perf record' to enable all markers and print a warning that 
> >>> the markers were in a partial state. This would result in consistent behaviour.
> >> Yes, makes sense.
> >>
> >>> Does it make sense to only enable some of the markers that alias on the same name? 
> >>> If not then maybe disallow that in perf probe - or change perf probe to do the 
> >>> same thing as perf record.
> >> 'perf probe' is doing that correctly. It fetches all events with given name from
> >> probe-cache and creates entries for them in uprobe_events.
> >>
> >> The problem is the 2-step process of adding probes and then recording,
> >> allowing users to select individual markers to record on.
> > So, the more streamlined one works for most people, i.e. just use perf
> > record, no need to perf probe anything. But, for people who "know what
> > they are doing", perf probe can be used first to control exactly which
> > SDT probes one wants in place, and then those will be used.
> >
> > We need to make sure that when processing the file there is information
> > that says which probes were in place and enabled in the record session,
> > tho. Is that possible?
> >
> >>> I.e. this is IMHO an artificial problem that users should not be exposed to and 
> >>> which can be solved by tooling.
> >>>
> >>> In particular if it's possible to enable only a part of the markers then perf 
> >>> record not continuing would be a failure mode: if for example a previous perf 
> >>> record session segfaulted (or ran out of RAM or was killed in the wrong moment or 
> >>> whatever) then it would not be possible to (easily) clean up the mess.
> >> Agreed. We need to make this more robust.
> > Right, disambiguating a 'probes left by a session that did auto-probing'
> > from a 'hey, those probes are there intentionally, just use those' is
> > important.
> >
> >>>> Not allowing 'perf probe' for SDT event will solve all such issues.
> >>>> Also it will make user interface simple and consistent. Other current
> >>>> tooling (systemtap, for instance) also do not allow probing individual
> >>>> markers when there are multiple markers with the same name.
> >>> In any case if others agree with your change in UI flow too then it's fine by me, 
> >>> but please make it robust, i.e. if perf record sees partially enabled probes it 
> >>> should still continue.
> >> @Masami, can you please provide your thoughts as well.
> > Yeah, if technically possible to allow both variants, we should leave
> > it up to users to decide what is best?
> >
> > I.e. most people will do auto-probing, not using 'perf probe' at all,
> > documentation should state the pitfalls in doing so.
> >
> > So, after writing the above, perhaps we should warn the user that
> > pre-existing probes are being used, as this will be the odd case?
> >
> > The normal flow will be just using perf record with SDT probes, that
> > will auto-probe them and remove on exit, or better drop a reference to
> > them, as simultaneous use also needs to be covered?
> >
> > - Arnaldo
> >