Messages in this thread | | | From | Joel Fernandes <> | Subject | Re: [RFC PATCH 00/11] Reviving the Proxy Execution Series | Date | Wed, 19 Oct 2022 08:23:39 -0400 |
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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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