Martin Bligh wrote:
Ingo Molnar wrote:
* Martin Bligh <[email protected]> wrote:
i very much agree that they should become as fast as possible. So
to rephrase the question: can we make dynamic tracepoints as fast
(or nearly as fast) as static tracepoints? If yes, should we care
about static tracers at all?
Depends how many nops you're willing to add, I guess. Anything, even
the static tracepoints really needs at least a branch to be useful,
IMHO. At least for what I've been doing with it, you need to stop
the data flow after a while (when the event you're interested in
happens, I'm using it like a flight data recorder, so we can go back
and do postmortem on what went wrong). I should imagine branch
prediction makes it very cheap on most modern CPUs, but don't have
hard data to hand.
only 5 bytes of NOP are needed by default, so that a kprobe can
insert a call/callq instruction. The easiest way in practice is to
insert a _single_, unconditional function call that is patched out to
NOPs upon its first occurance (doing this is not a performance issue
at all). That way the only cost is the NOP and the function parameter
preparation side-effects. (which might or might not be significant -
with register calling conventions and most parameters being readily
available it should be small.)
note that such a limited, minimally invasive 'data extraction point'
infrastructure is not actually what the LTT patches are doing. It's
not even close, and i think you'll be surprised. Let me quote from
the latest LTT patch (patch-2.6.17-lttng-0.5.108, which is the same
version submitted to lkml - although no specific tracepoints were
submitted):
OK, I grant you that's pretty scary ;-) However, it's not the only way
to do it. Most things we're using write a statically sized 64-bit event
into a relayfs buffer, with a timestamp, a minor and major event type,
and a byte of data payload.
believe it or not, this is inlined into: kernel/sched.c ...
'enuff said. LTT is so far from being even considerable that it's not
even funny.
Particularly if we're doing more complex things like that, I'd agree
that the overhead of doing the out of line jump is non-existant by
comparison. Even with the relayfs logging alone, perhaps the jump is
not that heavy ... hmmm.
If we put the NOPs in (at least as an option on some architectures)
from a macro, you don't really need the full kprobes implemented to
to tracing, even ... just overwrite the nops with a jump, so presumably
would be easier to port. However, not sure how local variable data
is specified in that case ... perhaps the kprobes guys know better.
Most of the complexity seemed to be with relocating existing code
because you didn't have nops.
With kprobes one can place probes anywhere you want but the ones placed
in the middle of the function are not maintainable because they are tied
to a location in the code. Having a NOP leaves a maintainable address
that we can hook into when needed.
AFAIK writing a portable code for using local variables is not easy
without using DWARF information, hence we don't handle that in kprobes.
Jprobes is a special case where you can have access to function
arguments at the function entry point. SystemTap can be used to specify
probes anywhere in the function and local variables can also be used in
the probe handlers. The problem still is maintainability as probes are
specified using line numbers.
To me, the main thing is to have hooks for the at least some of the
basic needs maintained in-kernel - from the dtrace paper Val pointed
me to, that seems to be exactly what they do too, and it integrates
with the newly added dynamic ones where necessary.
Once we have these static markers one can use both dynamic probes and
static probes intermixed getting best of both worlds as Frank
demonstrated in OLS.
Here are couple of proposals that were discussed in the systemtap
mailing list in how to specify static markers, we could use these ideas
with the rest in deciding on a maker proposal.
http://sources.redhat.com/ml/systemtap/2006-q3/msg00273.html
http://sourceware.org/ml/systemtap/2005-q4/msg00415.html
Plus I hate the
whole awk thing, and general complexity of systemtap, but we can
probably avoid that easily enough - either the embedded C option
you mentioned, or just a different definiton for the same hook macros
under a config option.
So perhaps it'll all work. Still need a little bit of data maintained
in tree though.
For placing probes at the begin and end of function we don't really need
markers as function boundary works as a marker.
I think we only need markers in few places where an important decision
is made in the middle of a function.
M.
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