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

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Message-ID: <lnk2cwutkjihjie4cdvpitbymdp25pyeurf3pegkvzlvrwdwwq@faeqbabjlwpq>
Date: Fri, 15 Nov 2024 15:42:52 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: 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>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Florian Schmaus <flo@...kplace.eu>
Subject: Re: [PATCH 2/2] bcachefs: Set rebalance thread to SCHED_BATCH and
 nice 19

On Fri, Nov 15, 2024 at 09:26:56AM +0100, Florian Schmaus wrote:
> On 15/11/2024 06.43, Kent Overstreet wrote:
> > On Thu, Nov 14, 2024 at 10:06:48PM +0100, Florian Schmaus wrote:
> > > Set the rebalance thread's scheduling class to BATCH, which means it
> > > could experience a higher scheduling latency. However, it reduces
> > > preemption events of running threads.
> > > 
> > > And while the rebalance thread is ually not compute bound, it does
> > > cause a considerable amount of I/O. By increasing its nice level from
> > > 0 to 19 we also implicitly reduce the thread's best-effort I/O
> > > scheduling class level from 4 to 7. Therefore, the rebalance thread's
> > > I/O operations will be deprioritized over standard I/O operations.
> > 
> > Is there a patch 1/2?
> 
> Sorry, patch 1/2 was unfortunately not send to linux-bcachefs@. You can find
> it at
> 
> https://lore.kernel.org/lkml/20241114210649.71377-1-flo@geekplace.eu/

2/2: https://lore.kernel.org/lkml/20241114210649.71377-2-flo@geekplace.eu/

Ingo, for sanity could we keep these two patches together? your tree or
mine is fine with me, if patch 1/2 is acceptable to you

Florian also had another idea I wanted to mention for giving userspace
control over sched policy - exposing the pids of our
rebalance/copygc/etc. threads in /sys/fs/bcachefs.

I'm liking it, do we have precedent elsewhere in the kernel for that?