lkml.org 
[lkml]   [2012]   [Jul]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] add blockconsole version 1.1
Date
On Tuesday 24 Jul 2012 15:38:22 Jörn Engel wrote:
> On Tue, 24 July 2012 09:01:16 +0100, Tvrtko Ursulin wrote:
> > On Monday 23 Jul 2012 21:02:30 Jörn Engel wrote:
> > > On Mon, 23 July 2012 15:33:16 +0100, Tvrtko Ursulin wrote:
> > > > On Thursday 12 Jul 2012 18:46:34 Jörn Engel wrote:
> > At the very least block console does not work from interrupt context
> > while netconsole does, right? Also netconsole does things to try and
> > work around low memory situations. Things like that I think would be
> > useful additions to documentation.
>
> Blockconsole does work from interrupt context. It has buffers for 1MB
> worth of data. Until those fill up, it only does a memcpy and
> schedules a workqueue for writeback. If you panic, it will do the
> writeback immediately. While I wouldn't believe this to always work,
> I have yet to see a confirmed failure case.

As far as I know there is nothing like netpoll in the block layer so it has to
be a lot less reliable than netconsole. Especially with delaying write out to
a workqueue. Anyway, I am not arguing, just saying in my opinion those caveats
are worth documenting.

> Blockconsole itself has no allocations in the write path, so it should
> be unaffected by low memory situation. The underlying driver and
> block layer code may well be.

Same thing.

> > > > Also, and I haven't checked what the swap format is, if it could
> > > > somehow be integrated together that could be useful.
> > >
> > > That appears to be slightly less likely than crossbreeding a rabbit
> > > with a chicken. Is there something obvious I have missed?
> >
> > I was thinking how swap space is always there and is potentially much
> > faster to write to than a random USB stick - which could translate to
> > more reliable. Then it's a question of which storage subsystem (libata
> > vs. usb-storage) would work better in different oops/panic situations.
> > Again I tend to have less hope in USB based solutions - maybe it's my
> > bias from working in that area many years ago. So the idea of swap space
> > was that _if_ swap format could be extended to allocate a number of
> > blocks to use other than swap, then that area could be used by
> > blockconsole. Seemed like a convenient and potentially more reliable
> > solution to me, but as I said the latter may depend.
>
> In my systems swap is often absent. Plus, taking a few blocks a swap
> aside is in the end just partitioning in a new dress. So the argumen
> appears to boil down to using partitions again. The equivalent of
> swap files might be interesting, but can also be somewhat scary. So I
> would leave it to others to actually write the code - if they care.

I knew you'll pick me up on a new partitioning scheme. :) I just see it as
convenience. Whereas it is often not possible (or at least to much effort) to
create new partitions, swap if often around and potentially more reliable than
a random USB stick (considering the whole data path).

> Libata is fine, blockconsole can work on any block device.

My point was that it's reliability will differ depending on the block device
in use, which is unlike netconsole. Again I am not arguing against the
feature, but if you don't see things like these are worth documenting I give
up.

Tvrtko
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2012-07-25 11:01    [W:0.079 / U:0.340 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site