RE: Service core statistics MT safety

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>
> >>>>
> >>>>> From: Mattias Rönnblom [mailto:mattias.ronnblom@ericsson.com]
> >>>>> Sent: Monday, 27 June 2022 13.06
> >>>>>
> >>>>> Hi.
> >>>>>
> >>>>> Is it safe to enable stats on MT safe services?
> >>>>>
> >>>>> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-
> 4
> >>>>> 54445555731-6096fdb16385f88f&q=1&e=27b94605-d1e2-40b6-af6d-
> 9ebc54d
> >>>>>
> 5db18&u=https%3A%2F%2Fgithub.com%2FDPDK%2Fdpdk%2Fblob%2Fmain%
> 2Flib
> >>>>> %2Feal%2Fcommon%2Frte_service.c%23
> >>>>> L3
> >>>>> 6
> >>>>> 6
> >>>>>
> >>>>> It seems to me this would have to be an __atomic_add for this code
> >>>>> to produce deterministic results.
> >>>>
> >>>> I agree. The same goes for the 'calls' field.
> >>> The calling function does the locking.
> >>> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-
> 454
> >>> 445555731-5ce419f8bf9f9b76&q=1&e=27b94605-d1e2-40b6-af6d-
> 9ebc54d5db1
> >>>
> 8&u=https%3A%2F%2Fgithub.com%2FDPDK%2Fdpdk%2Fblob%2Fmain%2Flib
> %2Feal
> >>> %2Fcommon%2Frte_service.c%23L3
> >>> 98
> >>>
> >>> For more information you can look at:
> >>> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-
> 454
> >>> 445555731-ba0d1416f08856f0&q=1&e=27b94605-d1e2-40b6-af6d-
> 9ebc54d5db1
> >>>
> 8&u=https%3A%2F%2Fgithub.com%2FDPDK%2Fdpdk%2Fblob%2Fmain%2Flib
> %2Feal
> >>> %2Finclude%2Frte_service.h%23L
> >>> 120
> >>>
> >>
> >> What about the
> >> https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-
> 4544
> >> 45555731-b64334addc78c264&q=1&e=27b94605-d1e2-40b6-af6d-
> 9ebc54d5db18&
> >>
> u=https%3A%2F%2Fgithub.com%2FDPDK%2Fdpdk%2Fblob%2Fmain%2Flib%2
> Feal%2F
> >> common%2Frte_service.c%23L404
> >> call (for MT safe services)?
> >>
> >> There's no lock held there.
> > Good point.
> > This is the case where the service running in service cores is MT safe. However,
> the stats are incremented outside of the MT Safety mechanism employed by the
> service. So, yes, this and other updates in the function
> 'service_runner_do_callback' need to be updated atomically.
> 
> Maybe a better solution would be to move this to the core_state struct (and
> eliminate the "calls" field since the same information is already in the core_state
> struct). The contention on these cache lines will be pretty crazy for services with
> short run time (say a thousand cycles or less per call), assuming they are
> mapped to many cores.
That's one option, the structures are internal as well. With this option stats need to be aggregated which will not give an accurate view. But, that is the nature of the statistics. 

I am also wondering if these stats are of any use other than for debugging. Adding a capability to disable stats might help as well.

> 
> Idle service cores will basically do nothing else than stall waiting for these lines, I
> suspect, hampering the progress of more busy cores.