[fedora-arm] Fedora 20 for Raspberry Pi????

[Andy Green]

Gordan Bobic <gordan at bobich.net> wrote:
>On 12/30/2013 11:54 AM, Andy Green wrote:
>>
>>
>> Gordan Bobic <gordan at bobich.net> wrote:
>>> On 12/30/2013 09:58 AM, Andy Green wrote:
>>>>
>>>>
>>>> Gordan Bobic <gordan at bobich.net> wrote:
>>>>> On 12/27/2013 04:02 PM, Richard W.M. Jones wrote:
>>>>>> On Fri, Dec 27, 2013 at 09:53:54AM +0000, Gordan Bobic wrote:
>>>>>>> How is transparent alignment fixup going to give you back the
>>>>>>> performance you lose from accesses straddling cache lines?
>>>>>>
>>>>>> You can have structs straddling cache lines and causing
>performance
>>>>>> problems without alignment issues, or structs being packed too
>>> close
>>>>>> together causing false sharing again w/o alignment being
>involved.
>>>>>>
>>>>>> If alignment problems cause performance issues, then we should
>deal
>>>>>> with those performance problems.  If they don't, we shouldn't
>worry
>>>>>> about them.
>>>>>>
>>>>>> Rich.
>>>>>>
>>>>>> ObHack: I once worked with an architecture [68k-based VME
>hardware]
>>>>>> that not only faulted on unaligned access, but also on accesses
>of
>>>>> the
>>>>>> wrong *size* (eg. using a short-sized read instruction instead of
>a
>>>>>> word-sized read instruction).  Dealing with that nonsense
>involved
>>> a
>>>>>> lot of compiler-specific massaging of code and some inline
>assembly
>>>>> ...
>>>>>
>>>>> I'm very glad you mentioned compilers - this is in fact easily
>>> fixable
>>>>> at compiler level. Intel's ICC has an option to make all arrays
>and
>>>>
>>>> No, if your code takes the approach to cast the struct pointer into
>a
>>> byte stream, the struct pointer itself can be unaligned.
>>>
>>> It won't fix all cases, but it will fix a large chunk of them -
>perhaps
>>>
>>> enough of them to make fixing the remainder a tractable problem.
>>
>> It's already tractable, you're choosing not to engage with solving it
>upstream.
>
>I'll enumerate the instances of this next time I'm doing a RedSleeve 
>rebuild (might start this week when I resurrect my Koji farm of
>armv5tel 
>devices). Last time I checked the number of instances logged was in the
>
>hundreds - sufficiently high that I just gave up.

Yeah but that's hundreds of instances of one bug in one package or one instance of a hundred bugs in different packages?  If it's in a library it might show up in a few different processes but still be one bug.

If it's in glibc it might show up many times in one session different ways but still be one issue.

Did you try catching the sigbus or whatever you're getting in gdb?

-Andy

>>>> Your compiler can do nothing about that, you have to touch the
>>> members using bytewise accessors to be compatible with SoCs that
>don't
>>> fix up unaligned access properly.
>>>>
>>>>> structs always aligned to a boundary (up to 16 byte, IIRC). If GCC
>>> were
>>>>>
>>>>> to implement such a feature the problem could be made to go away
>>>>> without
>>>>> actually addressing the underlying cause of the problem. It might
>be
>>> a
>>>>> bodge, but since complete fix of the underlying problem isn't
>going
>>> to
>>>>> happen anyway, a good bodge would be a lot better than doing
>>> nothing.
>>>>
>>>> What's wrong with you sending patches to the upstream?
>>>
>>> Nothing apart from the amount of man-months it would take to
>>
>> Nonsense... a few years ago I made my own cross distro for an ARM9
>device without hardware fixup, entirely from source tarballs, and there
>were almost no alignment issues in the major projects.
>
>I did the same 18 months ago, and my experience was distinctly 
>different. Thankfully, with the kernel-level alignment fixup at least 
>building the distro was tractable.
>
>> I guess they will tend to start to increase since more people are
>using newer ARM SoC which all have hardware fixup - but you are the
>backpressure against that by providing patches for the real problems
>you found... at least if you care about it, you should be.
>
>A fair point well made, but I don't think we entirely agree on the
>scale 
>of the problem.
>
>Gordan