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

[Kevin Laatz <kevin.laatz@intel.com>]

On 19/09/2022 11:19, Konstantin Ananyev wrote:
> Hi everyone,
>
>> From: Anatoly Burakov <anatoly.burakov@intel.com>
>>
>> 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. 
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. 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.
2. Ring does not have callback support, meaning pipelined applications 
could not report lcore poll busyness telemetry with this approach. 
Eventdev is another driver which would be completely missed with this 
approach.

BR,
Kevin