linux-kernel - Re: Laptop shock detection and harddisk protection

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Message-ID: <41840b750809111325t30f8ffe2sa4572d401d43dc5a@mail.gmail.com>
Date:	Thu, 11 Sep 2008 16:25:22 -0400
From:	"Shem Multinymous" <multinymous@...il.com>
To:	"Tejun Heo" <tj@...nel.org>
Cc:	"Elias Oltmanns" <eo@...ensachen.de>,
	"Thomas Renninger" <trenn@...e.de>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"IDE/ATA development list" <linux-ide@...r.kernel.org>
Subject: Re: Laptop shock detection and harddisk protection

Hi Tejun,

On Thu, Sep 11, 2008 at 12:34 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello, Shem Multinymous.
>> Using the input device interface for the accelerometer (as done by
>> tp_smapi's hdaps + latest hdapsd) greatly reduces the number of
>> accelerometer-related timer interrupts on tickless kernels, as
>> measured by powertop. With syscall polling you have the kernal polling
>> the hardware at ~50Hz and then the userspace hdapsd polling the kernel
>> at ~50Hz. When they're out of phase so you can get up to 100
>> interrupts/sec. With an input device you're always at 50Hz. The phase
>> difference also induces a small extra delay in the shock handling
>> response.
>
> That reduction comes because input device supports poll and
> sysfs_notify_event() does about the same thing.  The uesrland daemon
> can just poll on a node and read data nodes when poll event on the
> node triggeres.

Agreed.
There's another issue with the current sysfs interface, though: hdapsd
needs to read (x,y,timestamp) tuples, whereas sysfs provides just x
and y in separate attributes which cannot be read atomically together.
We can add a sysfs file with "x y timestamp" readouts, though this is
unusual for sysfs (and certainly incompatible with hwmon).

> Unloading heads will be simple.  Just echoing timeout in ms to sysfs
> nodes, so I don't think it's a good idea to push out actual unloading
> to another process especially as fork doesn't inherit mlockall.

I had in mind another daemon listening for "unload now" events, so no
forking needed.
This second daemon might make sense if we push the logic of deciding
*which* disks to unload into userspace, since this logic is the same
for the ThinkPad style and the HP style.

> On a related note, is there any plan to merge tp_smapi to mainline?
> It seems you put a lot of work into it and I don't really see why it
> should stay out of tree.

The only issue I'm aware of is finding a reasonably-named maintainer.
On the technical side, the reviews on my lkml submission of
thinkpad_ec+hdaps seemed good and all technical comments are since
addressed. The code has been stable, well-tested and packaged by major
distros for years.

  Shem
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