linux-kernel - Re: [PATCH 2/2] bcachefs: set rebalance thread to SCHED

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Message-ID: <20250114144000.GU5388@noisy.programming.kicks-ass.net>
Date: Tue, 14 Jan 2025 15:40:00 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Christian Loehle <christian.loehle@....com>
Cc: Florian Schmaus <flo@...kplace.eu>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] bcachefs: set rebalance thread to SCHED_BATCH and
 nice 19

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.