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

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Date:	Thu, 31 May 2012 07:33:25 -0300
From:	Mauro Carvalho Chehab <mchehab@...hat.com>
To:	Borislav Petkov <bp@...64.org>
CC:	"Luck, Tony" <tony.luck@...el.com>,
	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

Em 31-05-2012 07:00, Borislav Petkov escreveu:
> On Wed, May 30, 2012 at 11:24:41PM +0000, Luck, Tony wrote:
>>>         u32 grain;              /* granularity of reported error in bytes */
>>> 				   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>>>> 			dimm->grain = nr_pages << PAGE_SHIFT;
>>
>> I'm not at all sure what we'll see digging into the chipset registers
>> like EDAC does - but we do have different granularity when reporting
>> via machine check banks.  That's why we have this code:
>>
>>                 /*
>>                  * Mask the reported address by the reported granularity.
>>                  */
>>                 if (mce_ser && (m->status & MCI_STATUS_MISCV)) {
>>                         u8 shift = MCI_MISC_ADDR_LSB(m->misc);
>>                         m->addr >>= shift;
>>                         m->addr <<= shift;
> 
> That's 64 bytes max, IIRC.
> 
>> in mce_read_aux().  In practice right now I think that many errors will
>> report with cache line granularity,
> 
> Yep.
> 
>> while a few (IIRC patrol scrub) will report with page (4K)
>> granularity. Linux doesn't really care - they all have to get rounded
>> up to page size because we can't take away just one cache line from a
>> process.
> 
> I'd like to see that :-)
> 
>>> @Tony: Can you ensure us that, on Intel memory controllers, the address
>>> mask remains constant at module's lifetime, or are there any events that
>>> may change it (memory hot-plug, mirror mode changes, interleaving 
>>> reconfiguration, ...)?
>>
>> I could see different controllers (or even different channels) having
>> different setup if you have a system with different size/speed/#ranks
>> DIMMs ... most systems today allow almost arbitrary mix & match, and the
>> BIOS will decide which interleave modes are possible based on what it
>> finds in the slots.  Mirroring imposes more constraints, so you will
>> see less crazy options. Hot plug for Linux reduces to just the hot add
>> case (as we still don't have a good way to remove DIMM sized chunks of
>> memory) ... so I don't see any clever reconfiguration possibilities
>> there (when you add memory, all the existing memory had better stay
>> where it is, preserving contents).
> 
> You're funny :-)
> 
>> Perhaps the only option where things might change radically is socket
>> migration ... where the constraint is only that the target of the
>> migration have >= memory of the source. So you might move from some
>> weird configuration with mixed DIMM sizes and thus no interleave, to a
>> homogeneous socket with matched DIMMs and full interleave. But from an
>> EDAC level, this is a new controller on a new socket ... not a changed
>> configuration on an existing socket.
> 
> Right, from the frequency of such events happening, it still sounds to
> me like the perfect place for the grain value is in sysfs.

Huh? Tony said that some errors report at 4K granularity while, others are at
cache line size. So, the granularity is a dynamic, per-error value.

Mapping a dynamic per-error field at sysfs is a huge mistake, as it would
require to change the sysfs value for every error report, and that race 
conditions will happen, if userspace is not fast enough to read sysfs for
every single event, before the next one happens.

I can't see any alternative to return a per-error field other than using
the same API for the error and for all per-error fields. So, the grain should
be part of the tracepoint.

Regards,
Mauro
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