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

From: Ravi Bangoria
Date: Mon Feb 20 2017 - 08:19:05 EST

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.

> 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.

>> 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.