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SubjectRe: Tracehooks in scheduler
(+ LKML)

Apologies forgot to CC the list.

On 04/07/19 18:52, Qais Yousef wrote:
> Hi Steve, Peter
>
> I know the topic has sprung up in the past but I couldn't find anything that
> points into any conclusion.
>
> As far as I understand new TRACE_EVENTS() in the scheduler (and probably other
> subsystems) isn't desirable as it intorduces a sort of ABI that can be painful
> to maintain.
>
> But for us to be able to test various aspect of EAS, we rely on some events
> that track load_avg, util_avg and some other metrics in the scheduler.
> Example of such patches that are in android and we maintain out of tree can be
> found here:
>
> https://android.googlesource.com/kernel/common/+/42903694913697da88a4ac627a92bbfdf44f0a2e
> https://android.googlesource.com/kernel/common/+/6dfaed989ea4ca223f0913dfc11cdafd9664fc1c
>
> Dietmar and Quentin pointed me to a discussion you guys had with Daniel Bristot
> in the last LPC when he had a similar need. So it is something that could
> benefit other users as well.
>
> What is the best way forward to be able to add tracehooks into the scheduler
> and any other subsystem for that matters?
>
> We tried using DECLARE_TRACE() to create a tracepoint which doesn't export
> anything in /sys/kernel/debug/tracing/events and hoped that we can use eBPF or
> a kernel module to attach to this tracepoint and access the args to inject our
> own trace_printks() but this didn't work. The glue logic necessary to attach
> to this tracepoint in a similar manner to how RAW_TRACEPOINT() in eBPF works
> isn't there AFAICT.
>
> I can post the full example if the above doesn't make sense. I am still
> familiarizing myself with the different aspects of this code as well. There
> might be support for what we want but I failed to figure out the magic
> combination to get it to work.
>
> If I got this glue logic done, would this be an acceptable solution? If not, do
> you have any suggestions on how to progress?
>
> Thanks
>
> --
> Qais Yousef

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