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SubjectRe: [RFC PATCH 00/11] Reviving the Proxy Execution Series
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> On Oct 19, 2022, at 7:43 AM, Qais Yousef <qyousef@layalina.io> wrote:
>
> On 10/17/22 02:23, Joel Fernandes wrote:
>
>> I ran a test to check CFS time sharing. The accounting on top is confusing,
>> but ftrace confirms the proxying happening.
>>
>> Task A - pid 122
>> Task B - pid 123
>> Task C - pid 121
>> Task D - pid 124
>>
>> Here D and B just spin all the time. C is lock owner (in-kernel mutex) and
>> spins all the time, while A blocks on the same in-kernel mutex and remains
>> blocked.
>>
>> Then I did "top -H" while the test was running which gives below output.
>> The first column is PID, and the third-last column is CPU percentage.
>>
>> Without PE:
>> 121 root 20 0 99496 4 0 R 33.6 0.0 0:02.76 t (task C)
>> 123 root 20 0 99496 4 0 R 33.2 0.0 0:02.75 t (task B)
>> 124 root 20 0 99496 4 0 R 33.2 0.0 0:02.75 t (task D)
>>
>> With PE:
>> PID
>> 122 root 20 0 99496 4 0 D 25.3 0.0 0:22.21 t (task A)
>> 121 root 20 0 99496 4 0 R 25.0 0.0 0:22.20 t (task C)
>> 123 root 20 0 99496 4 0 R 25.0 0.0 0:22.20 t (task B)
>> 124 root 20 0 99496 4 0 R 25.0 0.0 0:22.20 t (task D)
>>
>> With PE, I was expecting 2 threads with 25% and 1 thread with 50%. Instead I
>> get 4 threads with 25% in the top. Ftrace confirms that the D-state task is
>> in fact not running and proxying to the owner task so everything seems
>> working correctly, but the accounting seems confusing, as in, it is confusing
>> to see the D-state task task taking 25% CPU when it is obviously "sleeping".
>>
>> Yeah, yeah, I know D is proxying for C (while being in the uninterruptible
>> sleep state), so may be it is OK then, but I did want to bring this up :-)
>
> I seem to remember Valentin raised similar issue about how userspace view can
> get confusing/misleading:
>
> https://www.youtube.com/watch?v=UQNOT20aCEg&t=3h21m41s

Thanks for the pointer! Glad to see the consensus was that this is not acceptable.

I think we ought to write a patch to fix the accounting, for this series. I propose adding 2 new entries to proc/pid/stat which I think Juri was also sort of was alluding to:

1. Donated time.
2. Proxied time.

User space can then add or subtract this, to calculate things correctly. Or just display them in new columns. I think it will also actually show how much the proxying is happening for a use case.

Thoughts?

Thanks.


> Cheers
>
> --
> Qais Yousef

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