Messages in this thread | | | Date | Fri, 15 Sep 2006 10:57:17 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108 |
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On 15 Sep 2006 13:08:29 -0400 fche@redhat.com (Frank Ch. Eigler) wrote:
> Alan Cox <alan@lxorguk.ukuu.org.uk> writes: > > > [...] > > > > > > prepare_arch_switch(rq, next); > > > + TRACE_SCHEDCHANGE(prev, next); > > > prev = context_switch(rq, prev, next); > > > barrier(); > > > > The gdb debug data lets you find each line and also the variable > > assignments (except when highly optimised in some cases). [...] > > Unfortunately, variables and even control flow are quite regularly > made non-probe-capable by modern gcc. Statement boundaries and > variables are not preserved. There is an arms race within gcc to both > improve code optimization and its own "reverse-engineering" debugging > data generation, and the former is always ahead. > > The end result is that there are many spots that we'd like to probe in > systemtap, but can't place exactly or extract all the data we'd like. > Really.
Useful info, thanks.
> There are also spots that for other reasons cannot tolerate a fully > dynamic kprobes-style probe: > > - where 1000-cycle int3-dispatching overheads too high
Is that still true of the recent kprobes "boosting" changes?
> - in low-level code such as fault handling or locking, that, if probed > dynamically, could entail infinite regress > - debugging information may not be available > > This is the reason why I'm in favour of some lightweight event-marking > facility: a way of catching those points where dynamic probing is not > sufficiently fast or dependable.
OK.
> > [...] > > All we appear to lack is systemtap ability to parse debug data so it can > > be told "trace on line 9 of sched.c and record rq and next" > > Actually: > > #! stap > probe kernel.function("*@kernel/sched.c:9") { printf("%p %p", $rq, $next) } >
Really. That's impressive progress. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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