Messages in this thread | | | Date | Wed, 21 Sep 2022 16:08:19 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH 0/1] arm64: defconfig: Add Coresight as module |
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On Wed, Sep 21, 2022 at 03:05:34PM +0100, James Clark wrote: > As suggested by Catalin here's the change to add Coresight to defconfig. > > Unfortunately I don't think we should add CONFIG_CORESIGHT_SOURCE_ETM4X > which builds a few files until [1] is merged because of the overhead > of CONFIG_PID_IN_CONTEXTIDR. > > [1]: https://lore.kernel.org/lkml/20211021134530.206216-1-leo.yan@linaro.org/T/
I thought the overhead wasn't the problem, it's mostly negligible. We can probably save a few more cycles on the __switch_to() path by replacing several isb()s in those functions with a single one just before cpu_switch_to().
IIRC the issue is that unless a process runs in the root pid namespace, the actual pid written to contextidr is meaningless.
Now that you reminded me of that thread, I see three options (sorry, not entirely related to the defconfig updates):
1. Remove CONFIG_PID_IN_CONTEXTIDR and corresponding code completely, find other events to correlate the task with the trace.
2. Always on CONFIG_PID_IN_CONTEXTIDR (we might as well remove the Kconfig entry). This would write the root pid namespace value (task_pid_nr()).
3. Similar to (2) but instead write task_pid_nr_ns(). An alternative here is to write -1 if the task is not in the root pid namespace.
Strong preference for (1).
-- Catalin
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