linux-kernel - Is congestion broken?

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Message-ID: <20190923111900.GH15392@bombadil.infradead.org>
Date:   Mon, 23 Sep 2019 04:19:00 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     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, Jens Axboe <axboe@...nel.dk>,
        Omar Sandoval <osandov@...com>, Ming Lei <ming.lei@...hat.com>
Subject: Is congestion broken?


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?
> 
>