Date: Thu, 10 Jul 2008 10:22:08 -0400
From: "Frank Ch. Eigler" <fche@redhat.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>, systemtap@sourceware.org
Subject: Re: [RFC] simple dprobe like markers for the kernel
Message-ID: <20080710142208.GC1213@redhat.com>
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Hi -

On Thu, Jul 10, 2008 at 08:49:54AM -0500, James Bottomley wrote:
> [...]
> > Another disadvantage is one that came up earlier when markers were
> > initially thought up: that something so invisible to the compiler (no
> > code being generated in the instruction stream, after optimization,
> > may be impossible to locate: not just the statement but also the
> > putative parameters.
> 
> Actually, I listed that one as an advantage.  But, in order to be
> completely zero impact, the probe cannot interfere with optimisation,
> and so you run the risk of having the probe point do strange things
> (like it's in the middle of a loop that gets unrolled) or that the
> variables you want to advertise get optimised away.
> 
> All of this is mitigated by correct selection of the probe points and
> the variables.

Well, you can test your theory: replace some "tracepoints" or markers
or printk's with this, and see if systemtap (or gdb) can get at the
same data.

When "correct selection" is a function of any particular compiler's
optimization algorithms, it will be difficult for a human programmer
to get it right.


> > Long ago, someone proposed inserting an asm("nop") mini-markers into
> > the instruction stream, which could then be used as an anchor to tie a
> > kprobe to, so that would solve the statement-location problem.
> 
> But you don't need a nop ... you just need a line number.

That's *if* the line number ends up being resolvable back to a PC.  In
fact, since there is no code emitted for it, that particular line
number will not actually appear in DWARF line records.


> > But it doesn't help assure that the parameters will be available in
> > dwarf, so someone else proposed adding another asm that just asks the
> > parameters to be evaluated and placed *somewhere*.  Each asm input
> > constraint was to be the loosest possible, so as to not force the
> > compiler to put the values into registers (and evict their normal
> > tracing-ignorant tenants).
> 
> Actually, it does.  Assuming the probe is placed in the code by someone
> who knows what they're doing and is using it, you can ensure that what
> you're advertising actually exists.  [...]

You misunderstood - I am not talking about whether the variables exist
in the context of the source code.  The question is which of those
variables still exist, live & addressable, in the machine code and
execution state.  You may be surprised to what extent compiler
optimizations disrupt a simple source-level reading of the situation.
 

> > So that's roughly how we arrived at recent markers.  They expose to
> > the compiler the parameters, but arrange not to evaluate them unless
> > necessary.  The most recent markers code patches nops over most or all
> > the hot path instructions, so there is no tangible performance impact.
> 
> Yes there are.  There are actually two performance impacts:
> 
>      1. The nops themselves take cycles to execute ... small, granted,
>         but it adds up with lots of probe points
>      2. The probes interfere with optimisation since to replace them
>         with a function call, they must be barriers. [...]

That's why I qualified it with "tangible".  Please confirm your
intuition about these costs.


- FChE
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