On 2024-12-11 05:32, Alexander Krabler via lttng-dev wrote:
Hi Christophe and Kienan,
Internal
On 12/2/24 4:17 PM, Christophe Bédard via lttng-dev wrote:
I did the same thing a while ago, i.e., trigger tracepoints on
malloc/free/etc. using liblttng-ust-libc-wrapper and collect userspace
callstack information (so that the indirect calls to malloc/free can be
removed from an application).
...
Thank you for the information and links!
I have looked into it, seems promising.
On 12/10/24 11:41 AM, Kienan Stewart via lttng-dev wrote:
Depending on the type of information you require, it could be possible
to use the instruction-pointer (ip) context[1], the statedump[2] and/or
lttng-ust-dl[3] for base addresses, and the symbol table in-order to
resolve ip -> symbol name in the offline analysis phase with the help of
babeltrace's debug-info plugin[4]. While a bit more work for the
analysis, this process reduces the impact of the tracing on the program
at run-time.
Thanks for the hints.
This would be possible, but that would give me only the instruction pointer of
the top-level stack frame.
If malloc and free calls are wrapped inside of functions (e.g. think about
std::string, std::unique/shared_ptr for C++),
I would only see that it's called from that specific type.
But, for me, it's important to see the context, too, where the string/smart
pointer is used.
Another approach then is to add tracepoints within your C++ standard
library functions. Then you can use the caller ip of _those_ functions
and pass them to the tracepoint as "ip" context override. Here is how
it is done e.g. in src/lib/lttng-ust-libc-wrapper/lttng-ust-malloc.c :
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TP_IP_PARAM ip <------ this.
#include "ust_libc.h"
src/lib/lttng-ust-libc-wrapper/ust_libc.h :
LTTNG_UST_TRACEPOINT_EVENT(lttng_ust_libc, malloc,
LTTNG_UST_TP_ARGS(size_t, size, void *, ptr, void *, ip),
LTTNG_UST_TP_FIELDS(
lttng_ust_field_integer(size_t, size, size)
lttng_ust_field_integer_hex(void *, ptr, ptr)
lttng_ust_field_unused(ip)
)
)
You can similarly specify which parameter end up being used to
override the caller ip sampling.
Are those C++ functions something that is under your control ?
Thanks,
Mathieu
Therefore I want to record the instruction pointer of every stackframe (or at least the
top N ones in order to be able to locate the "problem").
As stacktraces are only wanted/needed for specific events, I also consider
adding new tracepoints
like lttng_ust_libc:malloc_stacktrace to lttng-ust-libc-wrapper directly
recording the desired information into the trace buffer as an alternative
solution.
Best regards,
Alexander
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
