[PATCH 4/6] service: tweak cycle statistics semantics

[Morten Brørup]

> From: Mattias Rönnblom [mailto:mattias.ronnblom at ericsson.com]
> Sent: Tuesday, 6 September 2022 18.14
> 
> As a part of its service function, a service usually polls some kind
> of source (e.g., an RX queue, a ring, an eventdev port, or a timer
> wheel) to retrieve one or more items of work.
> 
> In low-load situations, the service framework reports a significant
> amount of cycles spent for all running services, despite the fact they
> have performed little or no actual work.
> 
> The per-call cycle expenditure for an idle service (i.e., a service
> currently without pending jobs) is typically very low. Polling an
> empty ring or RX queue is inexpensive. However, since the service
> function call frequency on an idle or lightly loaded lcore is going to
> be very high indeed, the service function calls' cycles adds up to a
> significant amount. The only thing preventing the idle services'
> cycles counters to make up 100% of the available CPU cycles is the
> overhead of the service framework itself.
> 
> If the RTE_SERVICE_ATTR_CYCLES or RTE_SERVICE_LCORE_ATTR_CYCLES are
> used to estimate service core load, the cores may look very busy when
> the system is mostly doing nothing useful at all.
> 
> This patch allows for an idle service to indicate that no actual work
> was performed during a particular service function call (by returning
> -EAGAIN). In such cases the RTE_SERVICE_ATTR_CYCLES and
> RTE_SERVICE_LCORE_ATTR_CYCLES values are not incremented.
> 
> The convention of returning -EAGAIN for idle services may in the
> future also be used to have the lcore enter a short sleep, or reduce
> its operating frequency, in case all services are currently idle.
> 
> This change is backward-compatible.
> 
> Signed-off-by: Mattias Rönnblom <mattias.ronnblom at ericsson.com>
> ---

This entire series contains a bunch of good improvements.

Returning -EAGAIN is a step in the right direction towards measuring CPU usage, and a great way to make it backwards compatible.

Series-Acked-by: Morten Brørup <mb at smartsharesystems.com>