Is congestion broken?

Matthew Wilcox
2019-09-23T11:19:11Z

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?
>
>