Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Fri, Nov 1, 2024 at 10:41 AM Josh Poimboeuf <[email protected]> wrote:
>
> On Thu, Oct 31, 2024 at 04:28:08PM -0700, Andrii Nakryiko wrote:
> > So all task_structs on the system using 104 bytes more, *permanently*
>
> Either way it's permanent, we don't know when to free it until the task
> struct is freed...
>

I'm not sure if we are arguing for the sake of arguing at this point
:) Yes, for *those tasks* for which we at least once requested stack
trace, that memory will stay, sure. But there are normally tons of
threads that are almost completely idle and/or use so little CPU, that
they won't ever be caught in the profiler, so their stack trace will
never be requested.

Sure, you can come up with a use case where you'll just go over each
task and ask for stack trace for each of them, but that's not a common
case.

So, sorry, but no, I don't agree that these are equivalent things.
Lazy memory allocation is a must, IMO.

> > and *unconditionally*, is not a concern
>
> Of course it's a concern, that's why we're looking for something
> better...
>
> > but lazy GFP_ATOMIC allocation when you actually need it is?
>
> We don't want to dip into the GFP_ATOMIC emergency reserves, those are
> kept for more important things.
>
> Actually, I think I can just use GFP_NOWAIT here.

Whatever semantics works for being called from NMI (even if it can fail).

>
> --
> Josh