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Date:   Mon, 23 Sep 2019 04:19:00 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Lin Feng <linf@wangsu.com>
Cc:     Michal Hocko <mhocko@kernel.org>, corbet@lwn.net,
        mcgrof@kernel.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        keescook@chromium.org, mchehab+samsung@kernel.org,
        mgorman@techsingularity.net, vbabka@suse.cz, ktkhai@virtuozzo.com,
        hannes@cmpxchg.org, Jens Axboe <axboe@kernel.dk>,
        Omar Sandoval <osandov@fb.com>, Ming Lei <ming.lei@redhat.com>
Subject: Is congestion broken?
Message-ID: <20190923111900.GH15392@bombadil.infradead.org>
References: <20190917115824.16990-1-linf@wangsu.com>
 <20190917120646.GT29434@bombadil.infradead.org>
 <20190918123342.GF12770@dhcp22.suse.cz>
 <6ae57d3e-a3f4-a3db-5654-4ec6001941a9@wangsu.com>
 <20190919034949.GF9880@bombadil.infradead.org>
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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?
> 
>