linux-kernel - Re: Is congestion broken?

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Date:   Mon, 23 Sep 2019 13:38:23 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>, Lin Feng <linf@...gsu.com>
Cc:     Michal Hocko <mhocko@...nel.org>, corbet@....net,
        mcgrof@...nel.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        keescook@...omium.org, mchehab+samsung@...nel.org,
        mgorman@...hsingularity.net, vbabka@...e.cz, ktkhai@...tuozzo.com,
        hannes@...xchg.org, Omar Sandoval <osandov@...com>,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: Is congestion broken?

On 9/23/19 5:19 AM, Matthew Wilcox wrote:
> 
> Ping Jens?
> 
> On Wed, Sep 18, 2019 at 08:49:49PM -0700, Matthew Wilcox wrote:
>> On Thu, Sep 19, 2019 at 10:33:10AM +0800, Lin Feng wrote:
>>> On 9/18/19 20:33, Michal Hocko wrote:
>>>> I absolutely agree here. From you changelog it is also not clear what is
>>>> the underlying problem. Both congestion_wait and wait_iff_congested
>>>> should wake up early if the congestion is handled. Is this not the case?
>>>
>>> For now I don't know why, codes seem should work as you said, maybe I need to
>>> trace more of the internals.
>>> But weird thing is that once I set the people-disliked-tunable iowait
>>> drop down instantly, this is contradictory to the code design.
>>
>> Yes, this is quite strange.  If setting a smaller timeout makes a
>> difference, that indicates we're not waking up soon enough.  I see
>> two possibilities; one is that a wakeup is missing somewhere -- ie the
>> conditions under which we call clear_wb_congested() are wrong.  Or we
>> need to wake up sooner.
>>
>> Umm.  We have clear_wb_congested() called from exactly one spot --
>> clear_bdi_congested().  That is only called from:
>>
>> drivers/block/pktcdvd.c
>> fs/ceph/addr.c
>> fs/fuse/control.c
>> fs/fuse/dev.c
>> fs/nfs/write.c
>>
>> Jens, is something supposed to be calling clear_bdi_congested() in the
>> block layer?  blk_clear_congested() used to exist until October 29th
>> last year.  Or is something else supposed to be waking up tasks that
>> are sleeping on congestion?

Congestion isn't there anymore. It was always broken as a concept imho,
since it was inherently racy. We used the old batching mechanism in the
legacy stack to signal it, and it only worked for some devices.

-- 
Jens Axboe