linux-kernel - Re: [PATCH v7 3/4] driver core: shut down devices asynchronously

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Date: Mon, 1 Jul 2024 12:57:40 -0500
From: stuart hayes <stuart.w.hayes@...il.com>
To: Christoph Hellwig <hch@....de>
Cc: linux-kernel@...r.kernel.org,
 Greg Kroah-Hartman <gregkh@...uxfoundation.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>, Lukas Wunner <lukas@...ner.de>,
 David Jeffery <djeffery@...hat.com>, Jeremy Allison <jallison@....com>,
 Jens Axboe <axboe@...com>, Sagi Grimberg <sagi@...mberg.me>,
 linux-nvme@...ts.infradead.org
Subject: Re: [PATCH v7 3/4] driver core: shut down devices asynchronously

On 6/27/2024 12:55 AM, Christoph Hellwig wrote:
> On Wed, Jun 26, 2024 at 02:46:49PM -0500, Stuart Hayes wrote:
>> Add code to allow asynchronous shutdown of devices, ensuring that each
>> device is shut down before its parents & suppliers.
>>
>> Add async_shutdown_enable to struct device_driver, and expose it via sysfs.
>> This can be used to view or change driver opt-in to asynchronous shutdown.
>> Only devices with drivers that have async_shutdown_enable enabled will be
>> shut down asynchronously.
>>
>> This can dramatically reduce system shutdown/reboot time on systems that
>> have multiple devices that take many seconds to shut down (like certain
>> NVMe drives). On one system tested, the shutdown time went from 11 minutes
>> without this patch to 55 seconds with the patch.
> 
> We discussed this before, but there is no summary of it and I of course
> forgot the conclusion:
> 
>   - why don't we do this by default?

It is done by default in this version, for devices whose drivers opt-in.

In the previous discussion, you mentioned that you thought "safe" was the
only sensible option (where "safe" was driver opt-in to async shutdown)...
that is the default (and only) option with this version.  Greg K-H also
requested opt-in as well, and suggested that "on" (driver opt-out) could
be removed.

>   - why is it safe to user enable it?

I guess it isn't necessarily safe, if there are any drivers that can't
handle their devices shutting down asynchronously. I thought it would be
nice to be able to enable driver opt-in from user space for testing, before
changing the default setting for the driver.

> 
>> + * @shutdown_after - used during device shutdown to ensure correct shutdown ordering.
> 
> Overly long line.
> 
>> +static ssize_t async_shutdown_enable_store(struct device_driver *drv, const char *buf,
> 
> .. and here.
> 
I can correct these lines. I thought that an 80 character line length limit
was no longer required, and saw another line a few lines above these that was
even longer... and the checkpatch script didn't flag it either.