RE: [PATCH v7 1/4] eal: add lcore poll busyness telemetry

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

Hi Kevin,

> >> Currently, there is no way to measure lcore poll busyness in a passive way,
> >> without any modifications to the application. This patch adds a new EAL API
> >> that will be able to passively track core polling busyness.
> >>
> >> The poll busyness is calculated by relying on the fact that most DPDK API's
> >> will poll for work (packets, completions, eventdev events, etc). Empty
> >> polls can be counted as "idle", while non-empty polls can be counted as
> >> busy. To measure lcore poll busyness, we simply call the telemetry
> >> timestamping function with the number of polls a particular code section
> >> has processed, and count the number of cycles we've spent processing empty
> >> bursts. The more empty bursts we encounter, the less cycles we spend in
> >> "busy" state, and the less core poll busyness will be reported.
> >>
> >> In order for all of the above to work without modifications to the
> >> application, the library code needs to be instrumented with calls to the
> >> lcore telemetry busyness timestamping function. The following parts of DPDK
> >> are instrumented with lcore poll busyness timestamping calls:
> >>
> >> - All major driver API's:
> >>    - ethdev
> >>    - cryptodev
> >>    - compressdev
> >>    - regexdev
> >>    - bbdev
> >>    - rawdev
> >>    - eventdev
> >>    - dmadev
> >> - Some additional libraries:
> >>    - ring
> >>    - distributor
> >>
> >> To avoid performance impact from having lcore telemetry support, a global
> >> variable is exported by EAL, and a call to timestamping function is wrapped
> >> into a macro, so that whenever telemetry is disabled, it only takes one
> >> additional branch and no function calls are performed. It is disabled at
> >> compile time by default.
> >>
> >> This patch also adds a telemetry endpoint to report lcore poll busyness, as
> >> well as telemetry endpoints to enable/disable lcore telemetry. A
> >> documentation entry has been added to the howto guides to explain the usage
> >> of the new telemetry endpoints and API.
> > As was already mentioned  by other reviewers, it would be much better
> > to let application itself decide when it is idle and when it is busy.
> > With current approach even for constant polling run-to-completion model there
> > are plenty of opportunities to get things wrong and provide misleading statistics.
> > My special concern - inserting it into ring dequeue code.
> > Ring is used for various different things, not only pass packets between threads (mempool, etc.).
> > Blindly assuming that ring dequeue returns empty means idle cycles seams wrong to me.
> > Which make me wonder should we really hard-code these calls into DPDK core functions?
> > If you still like to introduce such stats, might be better to implement it via callback mechanism.
> > As I remember nearly all our drivers (net, crypto, etc.) do support it.
> > That way our generic code   will remain unaffected, plus user will have ability to enable/disable
> > it on a per device basis.
> 
> Thanks for your feedback, Konstantin.
> 
> You are right in saying that this approach won't be 100% suitable for
> all use-cases, but should be suitable for the majority of applications.

First of all - could you explain how did you measure what is the 'majority' of DPDK applications?
And how did you conclude that it definitely work for all the apps in that 'majority'?
Second what bother me with that approach - I don't see s clear and deterministic way
for the user to understand would that stats work properly for his app or not.
(except manually ananlyzing his app code).

> It's worth keeping in mind that this feature is compile-time disabled by
> default, so there is no impact to any application/user that does not
> wish to use this, for example applications where this type of busyness
> is not useful, or for applications that already use other mechanisms to
> report similar telemetry.

Not sure that adding in new compile-time option disabled by default is a good thing...
For me it would be much more preferable if we'll go through a more 'standard' way here:
a) define clear API to enable/disable/collect/report such type of stats.
b) use some of our sample apps to demonstrate how to use it properly with user-specific code.
c) if needed implement some 'silent' stats collection for limited scope of apps via callbacks -
let say for run-to-completion apps that do use ether and crypto devs only.

 However, the upside for applications that do
> wish to use this is that there are no code changes required (for the
> most part), the feature simply needs to be enabled at compile-time via
> the meson option.
> 
> In scenarios where contextual awareness of the application is needed in
> order to report more accurate "busyness", the
> "RTE_LCORE_POLL_BUSYNESS_TIMESTAMP(n)" macro can be used to mark
> sections of code as "busy" or "idle". This way, the application can
> assume control of determining the poll busyness of its lcores while
> leveraging the telemetry hooks adding in this patchset.
> 
> We did initially consider implementing this via callbacks, however we
> found this approach to have 2 main drawbacks:
> 1. Application changes are required for all applications wanting to
> report this telemetry - rather than the majority getting it for free.

Didn't get it - why callbacks approach would require user-app changes?
In other situations - rte_power callbacks, pdump, etc. it works transparent to
user-leve code.  
Why it can't be done here in a similar way?

> 2. Ring does not have callback support, meaning pipelined applications
> could not report lcore poll busyness telemetry with this approach.

That's another big concern that I have:
Why you consider that all rings will be used for a pipilines between threads and should
always be accounted by your stats?
They could be used for dozens different purposes.
What if that ring is used for mempool, and ring_dequeue() just means we try to allocate
an object from the pool? In such case, why failing to allocate an object should mean
start of new 'idle cycle'?  

> Eventdev is another driver which would be completely missed with this
> approach.

Ok, I see two ways here:
- implement CB support for eventdev.
-meanwhile clearly document that this stats are not supported for eventdev  scenarios (yet).

> 
>