linux-kernel - Re: [PATCH] [RFC] vmscan.c: add a sysctl entry for controlling memory reclaim IO congestion

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Date:   Wed, 18 Sep 2019 20:49:49 -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: Re: [PATCH] [RFC] vmscan.c: add a sysctl entry for controlling
 memory reclaim IO congestion_wait length

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?