linux-kernel - Re: [PATCH] RAS: Add a tracepoint for reporting memory controller events

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Message-ID: <20120524164554.GM27063@aftab.osrc.amd.com>
Date:	Thu, 24 May 2012 18:45:54 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Mauro Carvalho Chehab <mchehab@...hat.com>
Cc:	Linux Edac Mailing List <linux-edac@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Aristeu Rozanski <arozansk@...hat.com>,
	Doug Thompson <norsk5@...oo.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] RAS: Add a tracepoint for reporting memory controller
 events

On Thu, May 24, 2012 at 01:13:17PM -0300, Mauro Carvalho Chehab wrote:
> > Why are we even exporting grain actually with each tracepoint
> > invocation? This is the granularity of reported error in bytes, and it,
> > as such, is statically assigned to a value in each driver. Userspace can
> > certainly figure out that value in a different way.
> 
> The API doesn't export the grain, except via the tracepoint/printk.

And this is exactly my question: if it is a static value which is set
once per driver, why do we have to issue it with _every_ tracepoint
invocation? Room in the per-cpu trace buffers is not for free.

> > But the more important question is: does the grain help us when handling
> > the error info in userspace?
> > 
> > It tells us that at this physical address with "grain" granularity we
> > had an error. So?
> 
> While a certain number of corrected errors that happened on different, sparsed,
> addresses may not mean a damaged memory, the same number of corrected errors
> happening at the same physical address/grain means that the DRAM chip that
> contains such address is damaged, so the corresponding DIMM needs to be 
> replaced.
> 
> So, the address/grain can be used by userspace algorithms to increase the
> probability that a DIMM is damaged.

I have no idea what you're saying here.

The DIMM can be pinpointed using the address only, why do you need the
grain too?

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