Understanding SOFT IRQ 1

[Mathieu Desnoyers]

On 2025-05-09 12:00, David Aldrich via lttng-dev wrote:
> Hi
> 
> I am just getting started with LTTng and Trace Compass. I can capture
> Kernel and UST events.
> 
> My purpose is to investigate why a periodic task, running on an
> isolated core, has occasional blips i.e. it's period (duration)
> extends.
> 
> I have inserted tracepoints to mark normal and extended durations. In
> Trace Compass the blip tracepoint is preceded by a brown state that is
> 'SOFT IRQ 1'. This extends down all cores. It lasts 105us and is
> likely to be the reason for the blip.
> 
> Please could someone explain to me what may be happening here and how
> can I find out the reason for this SOFT IRQ?

See include/linux/interrupt.h from the Linux kernel:

enum
{
         HI_SOFTIRQ=0,
         TIMER_SOFTIRQ,
         NET_TX_SOFTIRQ,
         NET_RX_SOFTIRQ,
         BLOCK_SOFTIRQ,
         IRQ_POLL_SOFTIRQ,
         TASKLET_SOFTIRQ,
         SCHED_SOFTIRQ,
         HRTIMER_SOFTIRQ,
         RCU_SOFTIRQ,    /* Preferable RCU should always be the last softirq */

         NR_SOFTIRQS
};

So SOFT IRQ 1 would be TIMER_SOFTIRQ.

Note that we have a TODO in lttng-modules:

lttng_dump_softirq_vec()

Implementing this by dumping this softirq table mapping numbers to names
would be a welcome contribution.

Thanks,

Mathieu

> 
> Best regards
> David


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