linux-kernel - Re: Abysmal HDD/USB write speed after sleep on a UEFI system

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Date:	Tue, 7 May 2013 13:05:00 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	"Artem S. Tashkinov" <t.artem@...os.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	psusi@...ntu.com
Subject: Re: Abysmal HDD/USB write speed after sleep on a UEFI system

On Tue, May 7, 2013 at 9:59 AM, Artem S. Tashkinov <t.artem@...os.com> wrote:
> May 7, 2013 09:25:40 PM,        Bjorn Helgaas  wrote:
>> [+cc Phillip]
>>
>>> I would suspect that Windows' complaint about the BIOS mucking up the MTRRs
>>> is likely the best hint. Likely Windows is detecting the problem and fixing
>>> it up on resume, thus it only complains about "reduced resume performance".
>>> If the MTRRs are messed up, then quite likely parts of RAM have become
>>> uncacheable, causing performance to get randomly slaughtered in various
>>> ways.
>>>
>>> From looking at the code it's not clear if we are checking/restoring the
>>> MTRR contents after resume. If not, maybe we should be.
>>
>>I agree; the MTRR warning is a good hint.  Artem?
>>
>>Phillip, I cc'd you because you have similar hardware and your
>>https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1131468 report is
>>slightly similar.  Have you seen anything like this "reduced
>>performance after resume" issue?  If so, can you collect /proc/mtrr
>>contents before and after suspending?
>>
>
> Like Robert Hancock correctly noted the Linux kernel lacks the code to check
> for MTTR changes after resume - I'm not a kernel hacker to write such a code ;-)
>
> Likewise there's no code to see if RAM pages have become uncacheable - i.e
> I've no idea how to check it either.
>
> According to /proc/mttr nothing changes on resume - only Windows detects
> the discrepancy between MTTR regions on resume. dmesg contains no warnings
> or errors (aside from usual ACPI SATA warnings - but they happen right on
> boot - so I highly doubt the ACPI or SATA layers can be the culprit, since USB
> exhibits a similar performance degradation).

I'm not sure if reading /proc/mtrr actually reads the registers out of
the CPU each time, or whether we just return the cached values we read
out during initial boot-up. If the latter, then this output isn't
really useful as there's no guarantee the values are still intact.

>
> In short, there's little to nothing that I can check.
>
> That bug report has nothing to do with my problem - my PC suspends and
> resumes more or less correctly - everything works (albeit some parts don't
> work as they should). That person also has a very outdated BIOS -  1904 from
> 08/15/2011. I wouldn't be surprised if BIOS update solved his problem.
>
> Best regards,
>
> Artem
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