On 19/09/2022 11:19, Konstantin Ananyev wrote:
Hi everyone,
From: Anatoly Burakov <anatoly.bura...@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
