Date:	Sat, 20 Nov 2010 10:00:22 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	huang ying <huang.ying.caritas@...il.com>
Cc:	Huang Ying <ying.huang@...el.com>, Len Brown <lenb@...nel.org>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	linux-acpi@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] Generic hardware error reporting mechanism

On Sat, Nov 20, 2010 at 10:52:56AM +0800, huang ying wrote:
> On Fri, Nov 19, 2010 at 9:56 PM,  <boris@...en8.de> wrote:
> > Yes, we need a generic error reporting format. Wait a second, this
> > error format structure looks very much like a tracepoint record to me -
> > it has common fields and record-specific fields. And we have all that
> > infrastructure in the kernel and yet you've decided to reimplement it
> > anew. Remind me again why?
> 
> You mean "struct trace_entry"? They are quite different on design. The
> record format in patch can incorporate multiple sections into one
> record, which is meaningful for hardware error reporting.

Nope, I mean a tracepoint and it can do all those things.

> And we do not need the fancy
> "/sys/kernel/debug/tracing/events/<xxx>/<xxx>/format", user space
> error daemon only consumes all error record it recognized and blindly
> log all other records.

Nobody said you needed that - the tracepoint contains all the
information you need.

[..]

> > - why do we need an
> > error field for _every_ device on the system? Looks like a bunch of
> > hoopla for only a few error types...
> 
> Because every device may report hardware errors, but not every device
> will do it. So just a pointer is added to "struct device" and
> corresponding data structure is only created when needed.
> 
> >> For example, the
> >> "error" directory for APEI GHES is as follow.
> >>
> >> /sys/devices/platform/GHES.0/error/logs
> >> /sys/devices/platform/GHES.0/error/overflows
> >> /sys/devices/platform/GHES.0/error/throttles
> >>
> >> Where "logs" is number of error records logged; "throttles" is number
> >> of error records not logged because the reporting rate is too high;
> >> "overflows" is number of error records not logged because there is no
> >> space available.
> >>
> >> Not all devices will report errors, so struct dev_herr_info and sysfs
> >> directory/files are only allocated/created for devices explicitly
> >> enable it.  So to enumerate the error sources of system, you just need
> >> to enumerate "error" directory for each device directory in
> >> /sys/devices.
> >
> > So how do you say which devices should report and which shouldn't report
> > errors, from userspace with a tool? Default actions? What if you forget
> > to enable error reporting of a device which actually is important?
> 
> In general all hardware errors should be reported and logged.

So why the need to enable/disable them? Why add all that code to
enable/disable them when all devices can report hw errors but not all
do it but all should do it... (I can go on forever). Do you see the
spaghetti statement?

> >> One device file (/dev/error/error) which mixed error records from all
> >> hardware error reporting devices is created to convey error records
> >> from kernel space to user space.  Because hardware devices are dealt
> >> with, a device file is the most natural way to do that.  Because
> >> hardware error reporting should not hurts system performance, the
> >> throughput of the interface should be controlled to a low level (done
> >> by user space error daemon), ordinary "read" is sufficient from
> >> performance point of view.
> >
> > Right, so no need for a daemon but who does the read? cat? How are you
> > going to collect all those errors? How do you enforce policies?
> 
> Some summary hardware error information can be put into printk. Error
> daemon is needed because we need not only log the the error but the
> predictive recovery. If you really have no daemon, cat can be used to
> log the error. I don't fully understand your words, you want to
> enforce policies without error daemon?

Sorry, I misread your original statement. So it is clear that we need
some kind of daemon to do some error post-processing.

> 
> > How do you inject errors?
> 
> We can use another device file to inject error, for example
> /dev/error/error_inject. Just write the needed information to this
> file. The format can be same as the error record defined as above,
> because it is highly extensible.

Same argument as above - you can do that with tracepoints without
duplicating functionality.

[.. ]

> >> A lock-less hardware error record allocator is provided.  So for
> >> hardware error that can be ignored (such as corrected errors), it is
> >> not needed to pre-allocate the error record or allocate the error
> >> record on stack.  Because the possibility for two hardware parts to go
> >> error simultaneously is very small, one big unified memory pool for
> >> hardware errors is better than one memory pool or buffer for each
> >> device.
> >
> > Yet another bloat winner. Why do we need a memory allocator for error
> > records?
> 
> The point is lockless not the memory allocator. The lockless memory
> allocator is not hardware error reporting specific, it can be used by
> other part of the kernel too.

Wait a second, are we talking about hardware errors or memory management
here? If you want to push your lockless memory allocator, send it in to
linux-mm and let the guys there look at it, but not in conjunction with
hw errors. That's like I'm going for a run and, btw, while I'm at it, I
can buy a coffee machine.

> > You either get a single critical error which shuts down the
> > system and you log it to persistent storage, if possible, or you work at
> > those uncritical errors one at a time.
> 
> Uncritical errors can be reported in NMI handler too. So we need
> lockless data structure for them.

Why? What's wrong with using a single struct on the stack? Are you
afraid that we might blow the NMI stack although NMIs don't nest?

[.. ]

Dude, let me save you the trouble: all everybody is trying to say is
that you can achieve all that with stuff already available in the
kernel. And HW errors are not that special to need a special subsystem
grown for them - you just need to handle them properly. The only thing
you should provide is the backend to persistent storage so that error
info can be put there - everything else is bloat.

-- 
Regards/Gruss,
    Boris.
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