Re: [PATCH sched/core] sched/rt: Fix RT_PUSH_IPI soft lockup loop

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 13 May 2026 15:42:14 -1000
Tejun Heo <tj@kernel.org> wrote:

> Hello,
> 
> On Wed, May 13, 2026 at 09:31:08PM -0400, Steven Rostedt wrote:
> > OK, this is what I was missing. The fact that the CPU was running a
> > softirq at the time that was running for a very long time that prevents
> > the schedule from happening.  
> 
> Right, although, in prod case, I don't think each softirq invocation is that
> long. It's maybe a few msecs, if that. However, there's a constant stream of
> them and if you slow down the CPU enough with IPIs, the CPU can't ever clear
> pending softirq although it only runs a short time each time it enters
> softirq.
> 
> > So if the current task running is SCHED_OTHER we still need to handle
> > the case where the next task is pinned, as it will cause a warning
> > again if it tries to move the fair task, especially since that doesn't
> > fix the overloading.
> > 
> > I think this requires a bit more complex fix. Perhaps if the current
> > task is fair and the next task is pinned, it needs to look for the task
> > after that one to move.  
> 
> I see. You know the code and history a lot better than I do. Wanna take
> over?

I could try, but there are still some things that I don't understand.
One is that to send more IPIs due to the RT pull request, there needs
to be RT tasks constantly sleeping. Is that happening in this use case?
Are the softirqs waking up RT tasks that run for a short time and go
back to sleep, causing the pull IPI to trigger again?

-- Steve