linux-kernel - Re: [PATCH] mm/backing-dev: show state of all bdi

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Message-Id: <20190723130729.522976a1f075d748fc946ff6@linux-foundation.org>
Date:   Tue, 23 Jul 2019 13:07:29 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] mm/backing-dev: show state of all bdi_writeback in
 debugfs

On Tue, 23 Jul 2019 15:49:32 +0300 Konstantin Khlebnikov <khlebnikov@...dex-team.ru> wrote:

> Currently /sys/kernel/debug/bdi/$maj:$min/stats shows only root bdi wb.
> With CONFIG_CGROUP_WRITEBACK=y there is one for each memory cgroup.
> 
> This patch shows here state of each bdi_writeback in form:
> 
> <global state>
> 
> Id: 1
> Cgroup: /
> <root wb state>
> 
> Id: xxx
> Cgroup: /path
> <cgroup wb state>
> 
> Id: yyy
> Cgroup: /path2
> <cgroup wb state>

Why is this considered useful?  What are the use cases.  ie, why should
we add this to Linux?

> mm/backing-dev.c |  106 +++++++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 93 insertions(+), 13 deletions(-)

No documentation because it's debugfs, right?

I'm struggling to understand why this is a good thing :(.  If it's
there and people use it then we should document it for them.  If it's
there and people don't use it then we should delete the code.