Messages in this thread | | | Date | Mon, 13 Nov 2023 18:21:06 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH RFC 04/10] perf: Introduce deferred user callchains |
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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.
> 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).
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