linux-kernel - Re: [PATCH 2/5] mm: correct calculation of cgroup wb's bg_thresh in wb_over_bg

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Date: Tue, 23 Jan 2024 10:43:28 -1000
From: Tejun Heo <tj@...nel.org>
To: Kemeng Shi <shikemeng@...weicloud.com>
Cc: willy@...radead.org, akpm@...ux-foundation.org,
	hcochran@...nelspring.com, mszeredi@...hat.com, axboe@...nel.dk,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] mm: correct calculation of cgroup wb's bg_thresh in
 wb_over_bg_thresh

On Wed, Jan 24, 2024 at 02:33:29AM +0800, Kemeng Shi wrote:
> The wb_calc_thresh will calculate wb's share in global wb domain. We need
> to wb's share in mem_cgroup_wb_domain for mdtc. Call __wb_calc_thresh
> instead of wb_calc_thresh to fix this.

That function is calculating the wb's portion of wb portion in the whole
system so that threshold can be distributed accordingly. So, it has to be
compared in the global domain. If you look at the comment on top of struct
wb_domain, it says:

/*
 * A wb_domain represents a domain that wb's (bdi_writeback's) belong to
 * and are measured against each other in.  There always is one global
 * domain, global_wb_domain, that every wb in the system is a member of.
 * This allows measuring the relative bandwidth of each wb to distribute
 * dirtyable memory accordingly.
 */

Also, how is this tested? Was there a case where the existing code
misbehaved that's improved by this patch? Or is this just from reading code?

Thanks.

-- 
tejun