On Wed, May 3, 2023 at 1:00 PM Tejun Heo [off-list ref] wrote:

Hello,

On Wed, May 03, 2023 at 09:48:55AM -1000, Tejun Heo wrote:

quoted

quoted

If so, that's the idea behind the context capture feature so that we
can enable it on specific allocations only after we determine there is
something interesting there. So, with low-cost persistent tracking we
can determine the suspects and then pay some more to investigate those
suspects in more detail.

Yeah, I was wondering whether it'd be useful to have that configurable so
that it'd be possible for a user to say "I'm okay with the cost, please
track more context per allocation". Given that tracking the immediate caller
is already a huge improvement and narrowing it down from there using
existing tools shouldn't be that difficult, I don't think this is a blocker
in any way. It just bothers me a bit that the code is structured so that
source line is the main abstraction.

Another related question. So, the reason for macro'ing stuff is needed is
because you want to print the line directly from kernel, right?

The main reason is because we want to inject a code tag at the
location of the call. If we have a code tag injected at every
allocation call, then finding the allocation counter (code tag) to
operate takes no time.

Is that
really necessary? Values from __builtin_return_address() can easily be
printed out as function+offset from kernel which already gives most of the
necessary information for triaging and mapping that back to source line from
userspace isn't difficult. Wouldn't using __builtin_return_address() make
the whole thing a lot simpler?

If we do that we have to associate that address with the allocation
counter at runtime on the first allocation and look it up on all
following allocations. That introduces the overhead which we are
trying to avoid by using macros.

Thanks.

--
tejun