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
Date: Thu, 10 Jul 2008 14:23:00 -0000 [thread overview]
Message-ID: <20080710142208.GC1213@redhat.com> (raw)
In-Reply-To: <1215697794.3353.5.camel@localhost.localdomain>
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
next prev parent reply other threads:[~2008-07-10 14:23 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1215638551.3444.39.camel__22002.9595810503$1215638656$gmane$org@localhost.localdomain>
2008-07-10 2:30 ` Frank Ch. Eigler
2008-07-10 13:51 ` James Bottomley
2008-07-10 14:23 ` Frank Ch. Eigler [this message]
2008-07-10 14:46 ` James Bottomley
2008-07-10 15:30 ` Theodore Tso
2008-07-10 15:57 ` James Bottomley
2008-07-10 18:20 ` Frank Ch. Eigler
2008-07-12 18:23 ` [PATCH] " James Bottomley
2008-07-12 20:05 ` [PATCH] systemtap: add parser for simple markers James Bottomley
2008-07-12 23:08 ` Frank Ch. Eigler
2008-07-14 16:28 ` [PATCH] simple dprobe like markers for the kernel Masami Hiramatsu
2008-07-14 22:03 ` James Bottomley
2008-07-09 21:23 [RFC] " James Bottomley
2008-07-10 3:40 ` Mathieu Desnoyers
2008-07-10 13:55 ` James Bottomley
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