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SubjectRe: [PATCH] Re: relayfs documentation sucks?
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On Sun, 2005-07-17 at 21:45 +0200, bert hubert wrote:
> On Sun, Jul 17, 2005 at 10:43:40AM -0500, Tom Zanussi wrote:
>
> > It is racey - in this mode, there's nothing to keep the kernel from
> > writing as much as it wants before the user side has a chance to read
> > any of it. The only way this can be used safely is to make sure the
> > kernel side isn't writing anything when the client is reading. This
> > would be typical of a flight-recording usage i.e. kernel writes a
> > bunch of data continuously, then stops and allows the client to read
> > whatever's in there.
>
> Or by numbering entries written out, when in flight-recording mode you
> wouldn't want to block the kernel.

Exactly! I've written a logging device to record data in the kernel
that a printk can't help with. I've used this in debugging inturrupts,
the scheduler, and high speed network packets. Where a printk to a
serial would just slow things down, and going to the network is too
expensive, and complex if you happen to be debugging the network. This
tool is called logdev (http://www.kihontech.com/logdev) and uses a ring
buffer that is like the relayfs overwrite mode. It can do printk like
records and when something goes wrong, I dump the buffer to the serial.
Or I have a user space program reading it from a device. I don't care
about anything that happened earlier, I want to only know what happened
up to the point I dumped the buffer. Lately, I've been usuing this with
Ingo's RT patch, and when the system locks up, I dump the buffer, and it
shows quite nicely where the lockup occurred, and why.

With Tom's help, I also have a version that uses relayfs as a backend in
overwrite mode. It's still a work in progress (so no web site yet!)
since there's some issues of using a singe buffer for multiple CPUs.
This helps in debugging race conditions since you need to see how events
interleave.

>
> > > In fact, it appears this might even happen in non-overwrite mode.
> >
> > It shouldn't ever be able to happen in non-overwrite mode - if it
> > did, it would be a bug. Can you be more specific as to how you see
> > this happening in this mode?
>
> Yeah - you're right. The misunderstanding is because in both cases
> (overwrite and non-overwrite) data is lost, except that in one case you lose
> old data, and in the other new data.
>
> It might be a good idea to document this as well.
>
> Btw, I've already uncovered interesting things using relayfs, but I still
> don't see the case for having it merged :-)

The reason I would like to see this merged, so kernel hackers don't need
to constantly write there own logging buffers everytime you need to
debug a complex area of the kernel.

-- Steve


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