Re: [RFC PATCH 0/5] Remove dependency on congestion_wait in mm/

[David Sterba]

On Mon, Sep 20, 2021 at 01:50:58PM +0100, Mel Gorman wrote:
> On Mon, Sep 20, 2021 at 12:42:44PM +0100, Matthew Wilcox wrote:
> > On Mon, Sep 20, 2021 at 09:54:31AM +0100, Mel Gorman wrote:
> > > This has been lightly tested only and the testing was useless as the
> > > relevant code was not executed. The workload configurations I had that
> > > used to trigger these corner cases no longer work (yey?) and I'll need
> > > to implement a new synthetic workload. If someone is aware of a realistic
> > > workload that forces reclaim activity to the point where reclaim stalls
> > > then kindly share the details.
> > 
> > The stereeotypical "stalling on I/O" problem is to plug in one of the
> > crap USB drives you were given at a trade show and simply
> > 	dd if=/dev/zero of=/dev/sdb
> > 	sync
> > 
> 
> The test machines are 1500KM away so plugging in a USB stick but worst
> comes to the worst, I could test it on a laptop.

There's a device mapper target dm-delay [1] that as it says delays the
reads and writes, so you could try to emulate the slow USB that way.

[1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/delay.html