[Bug 946] async flow rules affects pmd power management

[Bug 946] async flow rules affects pmd power management

[bugzilla]

https://bugs.dpdk.org/show_bug.cgi?id=946

            Bug ID: 946
           Summary: async flow rules affects pmd power management
           Product: DPDK
           Version: unspecified
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: eventdev
          Assignee: dev@dpdk.org
          Reporter: david.hunt@intel.com
  Target Milestone: ---

pmd power management functionality when using RTM transactions never enters
power saving mode. 

When testing using l3fwd-power to test the power_pmd functionality for DPDK
22.03 rc2, I noticed that the power saved in this DPDK version was
significantly reduced. 
I tried multiple BIOS/OS/kernel combinations, but then I tried DPDK 21.11,
where the problem went away. 

I then did a git bisect, which narrowed down the problem to commit
197e820c6685993ad75387de79707c81b5e1fc10 - "ethdev: bring in async queue-based
flow rules operations"

The patch has the effect of causing the RTM transaction in
rte_power_monitor_multi(), in the C file lib/eal/x86/rte_power_intrinsics.c, to
always fail at rte_xbegin, so we never get to the rte_power_pause, thereby we
never get to save any power. 
If we revert just this patch, then the rte_xbegin succeeds, and we get to call
rte_power_pause, and save power.

So this patch is now causing rte_power_monitor_multi() to fail. 

I'm not sure what the solution is, but it may be possible to re-work the async
flow rules patch, or it may be possible to re-work the pmd_power_management
code to avoid the impact of the flow rules patch. Investigation is ongoing.

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