Re: [PATCH 1/3] sched: add sched_task_call()

From: Jiri Kosina
Date: Fri Feb 20 2015 - 03:49:39 EST

Alright, so to sum it up:

- current stack dumping (even looking at /proc/<pid>/stack) is not
guaranteed to yield "correct" results in case the task is running at the
time the stack is being examined

- the only fool-proof way is to send IPI-NMI to all CPUs, and synchronize
the handlers between each other (to make sure that reschedule doesn't
happen in between on some CPU and other task doesn't start running in
the interim).
The NMI handler dumps its current stack in case it's running in context
of the process whose stack is to be dumped. Otherwise, one of the NMI
handlers looks up the required task_struct, and dumps it if it's not
running on any CPU

- For live patching use-case, the stack has to be analyzed (and decision
on what to do based on the analysis) in the NMI handler itself,
otherwise it gets racy again

Converting /proc/<pid>/stack to this mechanism seems like a correct thing
to do in any case, as it's slow path anyway.

The original intent seemed to have been to make this fast path for the
live patching case, but that's probably not possible, so it seems like the
price that will have to be paid for being able to finish live-patching of
CPU-bound processess is the cost of IPI-NMI broadcast.



Jiri Kosina
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