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Date: Tue, 14 Jan 2025 15:40:00 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Christian Loehle <christian.loehle@arm.com>
Cc: Florian Schmaus <flo@geekplace.eu>, Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
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	Valentin Schneider <vschneid@redhat.com>,
	Kent Overstreet <kent.overstreet@linux.dev>,
	linux-bcachefs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] bcachefs: set rebalance thread to SCHED_BATCH and
 nice 19
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 <28688fa8-718b-4ee6-8417-822efac8b603@arm.com>
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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.