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

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Message-ID: <ZxInC1U7WiB7FNkJ@wunner.de>
Date: Fri, 18 Oct 2024 11:14:51 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Michael Kelley <mhklinux@...look.com>,
	Stuart Hayes <stuart.w.hayes@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Martin Belanger <Martin.Belanger@...l.com>,
	Oliver O'Halloran <oohall@...il.com>,
	Daniel Wagner <dwagner@...e.de>, Keith Busch <kbusch@...nel.org>,
	David Jeffery <djeffery@...hat.com>,
	Jeremy Allison <jallison@....com>, Jens Axboe <axboe@...com>,
	Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Bert Karwatzki <spasswolf@....de>
Subject: Re: [PATCH v9 0/4] shut down devices asynchronously

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.

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

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)?

Thanks,

Lukas