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

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Date: Sun, 18 Feb 2024 10:35:41 +0800
From: Kemeng Shi <shikemeng@...weicloud.com>
To: Tejun Heo <tj@...nel.org>
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 2/9/2024 3:32 AM, Tejun Heo wrote:
> Hello, Kemeng.
> 
> On Thu, Feb 08, 2024 at 05:26:10PM +0800, Kemeng Shi wrote:
>> Hi Tejun, sorry for the delay as I found there is a issue that keep triggering
>> writeback even the dirty page is under dirty background threshold. The issue
>> make it difficult to observe the expected improvment from this patch. I try to
>> fix it in [1] and test this patch based on the fix patches.
>> Run test as following:
> 
> Ah, that looks promising and thanks a lot for looking into this. It's great
> to have someone actually poring over the code and behavior. Understanding
> the wb and cgroup wb behaviors have always been challenging because the only
> thing we have is the tracepoints and it's really tedious and difficult to
> build an overall understanding from the trace outputs. Can I persuade you
> into writing a drgn monitoring script similar to e.g.
> tools/workqueues/wq_monitor.py? I think there's a pretty good chance the
> visibility can be improved substantially.
Hi Tejun, sorry for the late reply as I was on vacation these days.
I agree that visibility is poor so I have to add some printks to debug.
Actually, I have added per wb stats to improve the visibility as we
only have per bdi stats (/sys/kernel/debug/bdi/xxx:x/stats) now. And I
plan to submit it in a new series.
I'd like to add a script to improve visibility more but I can't ensure
the time to do it. I would submit the monitoring script with the per wb
stats if the time problem does not bother you.
Thanks.
> 
> Thanks.
>