Re: [RFC PATCH 1/2] Marker probes in futex.c

[Mathieu Desnoyers]

* Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> On Tue, 2008-04-15 at 08:32 -0400, Mathieu Desnoyers wrote:
> > * Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> > > On Tue, 2008-04-15 at 17:23 +0530, K. Prasad wrote:
> > > 
> > > > +	trace_mark(futex_wait_called, "uaddr:%p fshared:%p val:%u "
> > > > +			"abs_time:%p bitset:%d",
> > > > +			uaddr, fshared, val, abs_time, bitset);
> > > 
> > > This is some seriuosly ugly looking gunk, why would we want stuff like
> > > that scattered across the code?
> > > 
> > 
> > I don't really see how it differs so much from printks, which kernel
> > developers are already familiar with.
> 
> Which never last longer than the debug session and are never exposed to
> enterprise kABI crap.
> 
> > > What is wrong with a few simple hooks like:
> > > 
> > >   trace_futex_wait(uaddr, fshares, val, abs_time, bitset);
> > > 
> > > and then deal with that.
> > > 
> > 
> > If any of your variable type changes, then you are exporting an unknown
> > data structure to user-space. _That_ would break a userspace tracer
> > whenever you change any of these kernel variables and you don't want
> > that.
> 
> trace_futex_wait()'s signature would make the compiler issue a complaint
> when the arguments suddenly changes type, no?
> 

Yes, but then you would have to create new code for each event you want
to trace. In the end, it would increase the icache footprint
considerably and would also make addition of new events cumbersome.

> > Exporting the field names and variable types helps to identify the
> > variables by their given names rather than their respective order.
> > Having the field type insures binary compatibility.
> > 
> > Clearly we can turn your trace_futex_wait(uaddr, fshares, val, abs_time,
> > bitset); into a trace_mark() with a simple define, and I don't see any
> > problem with that. I just want to make sure the event name, field names
> > and field types are exported, and this is done by markers. However, I
> > wonder why none of the kernel printk() are turned into specialized
> > defines to make the code "cleaner".. maybe it's because it is useful to
> > have everything declared in one spot after all.
> 
> I must be missing something here, printks don't need to look pretty
> because they never see the light of lkml. They get ripped out as soon as
> I understand what the heck happened.
> 

The thing is that the trace_marks really fills two purpose : they
extract information from the core kernel, which is meant to be shipped
on production systems so tracing tools can report what is happening on
the system and they also allow kernel hackers to add markers of their
own, so they can extract information about specific events they are
interested in along with the standard kernel instrumentation.

So, part of it is meant to be standard kernel information, part of it
can be used for debugging. And since the kernel code evolves through
time, it makes sense to have an infrastructure flexible enough to follow
these changes easily.

Your proposal is interesting though. We could keep the flexible markers
as the core infrastructure to declare static instrumentation and add
static inlines in C files to map function prototypes to markers, such
as:

static inline void trace_futex_wait(void *uaddr)
{
  trace_mark(futex_wait, "uaddr %p", uaddr);
}

static inline void trace_futex_wakeup(void *uaddr)
{
  trace_mark(futex_wakeup, "uaddr %p", uaddr);
}

But personnally I don't really see how these static inlines will look
cleaner than the trace_marks in them.


> > > Also, you seem to expose way too much futex internals; do you really
> > > need that? People will go use this marker crap like ABI and further
> > > restrain us from changing the code.
> > > 
> > 
> > Because we extract the field names and types, we can create tracer
> > plugins that would hook on field names rather than expect a specific
> > number of fields and fixed field types. It makes it possible to tolerate
> > missing fields pretty easily. But yes, tracer tools might have to be
> > adapted to internal kernel changes, since they must follow kernel
> > structure changes. However, staying as close as possible to a canonical
> > representation of event fields, staying far from the specific
> > implemetation, would help to lessen the inter-dependency. On the other
> > hand, it would probably hurt trace compactness and efficiency.
> 
> See, these tracer tools are my nightmare as member of an enterprise
> linux team. They'll make an already hard job even harder, no thanks!
> 
> At least reduce the hooks to the very bare minimum; only log when the
> mutex changes state; not this: we entered futex_wait; we exited with
> state crap.
> 
> Just log:
> 
>   futex: <uaddr> wait
>   futex: <uaddr> wakeup
> 
> And other fundamental events, the rest is just not needed.
> 

I totally agree with you on that. This is the approach I've used in the
LTTng instrumentation.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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