Re: [PATCH RFC 04/10] perf: Introduce deferred user callchains

[Namhyung Kim]

On Mon, Nov 13, 2023 at 9:21 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Nov 13, 2023 at 08:56:39AM -0800, Namhyung Kim wrote:
>
> > Ok, then BPF gets no user call stacks even with sframes.
>
> Well, you could obviously run another BPF program at return to user and
> stitch there.

Right, maybe some more work needed for bpf_get_stackid()
but it seems possible.

>
> > Ok, but ...
>
> > If the task is preempted, the call stack would be from another
> > task (if it also has the pending call stacks) and we need to
> > check which user call stack matches which kernel call stack.
> > So there's no guarantee we can just use adjacent samples.
>
> So upon requesting a user backtrace for the task it could request a
> token (from eg a global atomic u64 in the most crude case) and place
> this in the task_struct. The backtrace will emit this as forward
> reference for the user trace.
>
> Once the task_work generates the user stacktrace, it will again dump
> this token along with the user unwind, after which it will reset the
> token for this this task (to 0).

Yeah, I thought something like this first, but then I thought
"can we just use PID for this?"

Thanks,
Namhyung