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

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Message-ID: <ynauark4zdo6iu232kmraqd4guf3ovjd4dcnc7se73khypyc2y@xlnyl7kpwzx3>
Date: Tue, 14 Jan 2025 10:19:04 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Christian Loehle <christian.loehle@....com>
Cc: Florian Schmaus <flo@...kplace.eu>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, 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>, 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:
> On 1/14/25 12:47, Florian Schmaus wrote:
> > While the rebalance thread is isually not compute bound, it does cause
> 
> s/isually/usually
> 
> > a considerable amount of I/O. Since "reducing" the nice level from 0
> > to 19, also implicitly reduces the threads best-effort I/O scheduling
> > class level from 4 to 7, the reblance thread's I/O will be depriotized
> 
> s/depriotized/deprioritized/
> 
> > over normal I/O.
> > 
> > Furthermore, we set the rebalance thread's scheduling class to BATCH,
> > which means that it will potentially receive a higher scheduling
> > latency. Making room for threads that need a low
> > schedulinglatency (e.g., interactive onces).
> 
> s/schedulinglatency/
> I know nothing about bcachefs internals, but could this also be a problem?
> The rebalance thread might not run for O(second) or so? 

Actually that is a concern - six locks don't have priority inheritence.

For that matter, standard mutexes and rw semaphores don't either, just
the RT variants, which seems questionable to me...