linux-kernel - Re: [PATCH RFC v2 5/6] blk-throttle: support to destroy throtl

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Date: Tue, 16 Apr 2024 15:22:04 -1000
From: Tejun Heo <tj@...nel.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: axboe@...nel.dk, chenhuacai@...nel.org, josef@...icpanda.com,
	jhs@...atatu.com, svenjoac@....de, raven@...maw.net,
	pctammela@...atatu.com, qde@...cy.de, zhaotianrui@...ngson.cn,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	loongarch@...ts.linux.dev, cgroups@...r.kernel.org,
	yi.zhang@...wei.com, yangerkun@...wei.com,
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC v2 5/6] blk-throttle: support to destroy throtl_data
 when blk-throttle is disabled

Hello,

On Wed, Apr 17, 2024 at 09:13:34AM +0800, Yu Kuai wrote:
> > Probably a better interface is for unloading to force blk-throtl to be
> > deactivated rather than asking the user to nuke all configs.
> 
> I was thinking that rmmod in this case should return busy, for example,
> if bfq is currently used for some disk, rmmod bfq will return busy.
> 
> Is there any example that unloading will deactivate resources that users
> are still using?

Hmm... yeah, I'm not sure. Pinning the module while in use is definitely
more conventional, so let's stick with that. It's usually achieved by
inc'ing the module's ref on each usage, so here, the module refs would be
counting the number of active rules, I guess.

I'm not sure about modularization tho mostly because we've historically had
a lot of lifetime issues around block and blkcg data structures and the
supposed gain here is rather minimal. We only have a handful of these
policies and they aren't that big.

If hot path overhead when not being used is concern, lazy init solves most
of it, no?

Thanks.

-- 
tejun