linux-kernel - Re: [RFC PATCH 2/2] mm: introduce budgt control in readahead

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Date: Fri, 10 May 2024 05:14:10 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Zhaoyang Huang <huangzhaoyang@...il.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	"zhaoyang.huang" <zhaoyang.huang@...soc.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	Josef Bacik <josef@...icpanda.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-mm@...ck.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, steve.kang@...soc.com
Subject: Re: [RFC PATCH 2/2] mm: introduce budgt control in readahead

On Fri, May 10, 2024 at 11:06:14AM +0800, Zhaoyang Huang wrote:
> On Thu, May 9, 2024 at 8:40 PM Christoph Hellwig <hch@...radead.org> wrote:
> >
> > > +     unsigned long budgt = inode->i_sb->s_bdev ?
> > > +                     blk_throttle_budgt(inode->i_sb->s_bdev) : 0;
> >
> > The readahead code is used for all file systems, you can't just call
> > into block layer code here.
> >
> ok. I would like to know any suggestions on introducing throttle
> budget control into readahead which actually works as a negative
> feedback path. IMO, negative feedback is a good methodology which has
> been used in scheduler(EAS) and thermal control(IPA) and
> memory(MGLRU). I would like to suggest to have a try on have it work
> cross the boundary of memory and block layer.
> 
> vfs_read / page fault
> |
> readahead  <---------|
> |                               |
> aops->readpages    |
> |                               |
> block_layer------------

what you could do is have blk-throttle fail bios that are tagged as
readahead if we've hit the threshold?