Re: [PATCH] mm/backing-dev: show state of all bdi_writeback in debugfs

[Andrew Morton]

On Tue, 23 Jul 2019 15:49:32 +0300 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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.