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Date:   Mon, 13 Nov 2023 18:21:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Josh Poimboeuf <jpoimboe@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
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        Adrian Hunter <adrian.hunter@intel.com>,
        linux-perf-users@vger.kernel.org, Mark Brown <broonie@kernel.org>,
        linux-toolchains@vger.kernel.org
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).