A question regarding isolated cpus and software interrupts

[Mathieu Desnoyers]

On 2025-05-29 14:06, David Aldrich via lttng-dev wrote:
> Hi
> 
> My question isn't directly related to lttng but I would welcome some advice.
> 
> Our Linux application is showing 'blips' whereby there are occasional
> increases in processing times. This is for a user process that runs on
> an isolated core. Using lttng and TraceCompass I have found that the
> 'blip' sometimes coincides with a TIMER_SOFTIRQ event on the isolated
> core.
> 
> I was surprised that software interrupts may run on isolated cores. Is
> there a way of preventing this?

+CC Frederic Weisbecker.

How are the cores isolated on your system (exact kernel command line
options or commands issued) ? There are various degree of fine-tuning
to prevent disturbance from the tick (nohz full), scheduling, and
irqs that can be done explicitly. So depending on your specific
configuration, you may be missing isolation options that would
prevent the softirq from running on your isolated cpus.

Thanks,

Mathieu

> 
> Are there any techniques using lttng to gain more information related
> to the interrupt (e.g. what is causing it)?
> 
> Best regards
> David


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com