Date:	Fri, 13 Oct 2006 12:00:06 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Akinobu Mita <akinobu.mita@...il.com>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, ak@...e.de, Don Mullis <dwm@...r.net>
Subject: Re: [patch 0/7] fault-injection capabilities (v5)

On Sat, 14 Oct 2006 02:46:24 +0900
Akinobu Mita <akinobu.mita@...il.com> wrote:

> On Thu, Oct 12, 2006 at 02:26:25PM -0700, Andrew Morton wrote:
> 
> > You've presumably run a kernel with these various things enabled.  What
> > happens?  Does the kernel run really slowly?  Does userspace collapse in a
> > heap?  Does it oops and die?
> 
> I don't feel much slowness with STACKTRACE & FRAME_POINTER and
> enabling stacktrace filter. But with enabling STACK_UNWIND I feel
> big latency on X. (There are two type of implementation of stacktrace
> filter in it [1] using STACKTRACE with FRAME_POINTER, and [2] STACK_UNWIND)
> 
> I don't know why there is quite difference between simple STACKTRACE and
> STACK_UNWIND. I'm about to try to use rb tree rather than linked list in
> unwind.

umm, we've hit this before, recently - iirc it was making lockdep run
really slowly.

The new unwinding code is apparently really inefficient in some situations.
It wasn't expected that it would be called at a high frequency, except people
_do_ want to do that.

I forget the details, but I can cc people who have better memory.

> In order to prevent from breaking other userspace programs and to
> inject failures into only a specific code or process, process filter and
> stacktrace filter are available. Without using them the system would be
> almost unusable.
> 
> Now I'm stuck on the script in fault-injection.txt with random 700
> modules. This script just tries to load/unload for all available kernel
> modules. It usually get several oopses or CPU soft lockup now.  It
> seems that relatively large number of them involved around driver model
> (drivers/base/*).

I'm shocked ;)

> (I hope recent large number of error handle fixes
> especially by Jeff Garzik fix them)

We're getting there, but there's a lot more to do.

> > Also, one place where this infrastructure could be of benefit is in device
> > drivers: simulate a bad sector on the disk, a pulled cable, a timeout
> > reading from a status register, etc.  If that works well and is useful then
> > I can see us encouraging driver developers to wire up fault-injection in
> > the major drivers.
> > 
> > Hence it would be useful at some stage to go in and to actually do all this
> > for a particular driver.  As an example implementation for others to
> > emulate and as a test for the fault-injection infrastructure itself - we
> > may discover that new capabilities are needed as this work is done.
> > 
> > I wouldn't say this is an urgent thing to be doing, but it is a logical
> > next step..
> 
> Yes. I'm learning from md/faulty and scsi-debug module what they are
> doing and how to integrate such kind of features in general form.

Neato, thanks.  Please don't slow down!

It's the GFP_ATOMIC allocations which we care about, really.  Things like
GFP_KERNEL actually don't fail in the current implementation (unless the
caller got oom-killed), and all this check-for-allocation-failures work
we're doing is mainly for completeness, and in anticipation of changes in
the page allocator strategy.

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