Kernel memory depletion when application running with RT priority

Kernel memory depletion when application running with RT priority

[Nipun Gupta]

Hi,

I have a query related to memory depletion when running application with real-time priority.

We are running User-Space polling application (DPDK) with real time priority on one of our ARM based NXP platform with Kernel 5.4. We have set isolcpus for all the cores except core 0.
The application (DPDK l2fwd) is running on 'core 1', taking complete CPU using command (chrt --rr 99 l2fwd -c 2 -n 1 -- -p 0x20 -T 0).

We have observed that the kernel memory continuously decreases when any command like 'cat' is run on the kernel. These commands like 'cat' are running only on 'core 0' as isolcpus is used (also confirmed using ftrace).
Further observation is that when priority of ksoftirqd process running on 'core 1' is increased more than the application, the memory which was depleted gets freed.

Kmemleak has shown that after each command there is memory allocation done by the kernel in the system calls made by the commands.
This only gets freed when ksofirqd running on 'core 1' gets some time-slice to run.

Is this an expected behavior? My query is that why is ksofirqd process running on 'core 0' not freeing the memory; or is it waiting for ksofirqd process to run on 'core 1'?
In case anything is running with RT priority and consuming all the CPU, should ksoftirqd process have higher priority than that process?

Please note: We are not using RT kernel.

Thanks,
Nipun

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