Re: [PATCH 2/2] bcachefs: set rebalance thread to SCHED_BATCH and nice 19

[Kent Overstreet]

On Tue, Jan 14, 2025 at 03:40:00PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 14, 2025 at 01:29:04PM +0000, Christian Loehle wrote:
> 
> > I know nothing about bcachefs internals, but could this also be a problem?
> > The rebalance thread might not run for O(second) or so? 
> 
> SCHED_BATCH should not behave anything like that, mostly SCHED_BATCH
> tasks will not cause wakeup preemption. But otherwise they compete at
> the same level as everybody else.
> 
> Notably a BATCH and NORMAL task that are each while(1) loops will get
> the normal 50-50 distribution of time. It's just that when a NORMAL task
> is running, the waking of a BATCH task won't ever kick the NORMAL from
> the CPU, instead waiting for the tick to do so.
> 
> So a task that is IO heavy (as suggested here), that wakes a lot to
> issue further IO, will not immediately interrupt whatever is on the CPU,
> instead it waits until it gets selected through other means.

Ok, thanks for the clarification on SCHED_BATCH - that does make perfect
sense here.

Using nice as a proxy for io scheduler priority does still bug me a bit,
simply because having a working io scheduler is not something we've ever
been able to depend on. That's a hidden internal knob we'd like to make
visible. I wonder if there's some way the tooling could be improved.