Re: [PATCH v9 0/4] shut down devices asynchronously

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Oct 18, 2024 at 11:14:51AM +0200, Lukas Wunner wrote:
> On Fri, Oct 18, 2024 at 07:49:51AM +0200, Greg Kroah-Hartman wrote:
> > On Fri, Oct 18, 2024 at 03:26:05AM +0000, Michael Kelley wrote:
> > > In the process, the workqueue code spins up additional worker threads
> > > to handle the load.  On the Hyper-V VM, 210 to 230 new kernel
> > > threads are created during device_shutdown(), depending on the
> > > timing. On the Pi 5, 253 are created. The max for this workqueue is
> > > WQ_DFL_ACTIVE (256).
> [...]
> > I don't think we can put this type of load on all systems just to handle
> > one specific type of "bad" hardware that takes long periods of time to
> > shutdown, sorry.
> 
> Parallelizing shutdown means shorter reboot times, less downtime,
> less cost for CSPs.

For some systems, yes, but as have been seen here, it comes at the
offset of a huge CPU load at shutdown, with sometimes longer reboot
times.

> Modern servers (e.g. Sierra Forest with 288 cores) should handle
> this load easily and may see significant benefits from parallelization.

"may see", can you test this?

> Perhaps a solution is to cap async shutdown based on the number of cores,
> but always use async for certain device classes (e.g. nvme_subsys_class)?

Maybe, but as-is, we can't take the changes this way, sorry.  That is a
regression from the situation of working hardware that many people have.

thanks,

greg k-h