S3C2440A strange perfomance issue

[Mark Tinguely]

On 5/12/2011 12:31 PM, Владимир Фещенко wrote:
> 12.05.2011, 18:13, "Mark Tinguely"<marktinguely at gmail.com>:
>> On 5/12/2011 3:29 AM, Владимир Фещенко wrote:
>>
>>>   %cc -O0 -o test test.c
>>>
>>>   and execute it:
>>>
>>>   Code:
>>>
>>>   %/usr/bin/time -l ./test
>>>          120.16 real       119.44 user         0.16 sys
>>>         12177  involuntary context switches
>>>
>>>   time of execution is over 120 sec ... but after system reboot:
>>>
>>>   Code:
>>>
>>>   %/usr/bin/time -l ./test
>>>            2.85 real         2.55 user         0.25 sys
>>>           292  involuntary context switches
>>>
>>>   execution time is 2.85(!) sec, but it's not all!
>>>
>>>   Code:
>>>
>>>   %cat test>    /dev/null
>>>
>>>   %/usr/bin/time -l ./test
>>>          120.40 real       119.51 user         0.23 sys
>>>         12201  involuntary context switches
>>>
>>>   Once the file has been opened for reading (cat test>    /dev/null), execution time again increased to 120 sec (until the next reboot).
>>>
>>>   What is it?!
>> Sounds like the executable cache gets disabled on the executable page
>> that is also writable issue that we talked about year or so ago. If you
>> want a quick test, in pmap_fix_cache(), and exit the routine immediately
>> if the mapping is executable. I did some tracing, and there are cases
>> where this is not the correct solution - executable mappings that are
>> really shared and cache should be disabled. I have a idea level patch
>> for this but never tested it well enough.
>>
>> I also notice a huge jump in "involuntary context switch" counts in your
>> runs.
>>
>> --Mark Tinguely
> please, i want code sample for use pmap_fix_cache() :)
> may be this issue is caching problem ... but why 'open for read' make this effect?

If the page is still in a kernel mapping (mapped to do I/O) then the 
cache sharing rules are applied to turn off caching.

Below should bypass cache disabling if the page is executable. Quick 
test only:

*** arm/arm/pmap.c.orig Thu May 12 13:31:35 2011
--- arm/arm/pmap.c      Thu May 12 13:37:00 2011
*************** pmap_fix_cache(struct vm_page *pg, pmap_
*** 1313,1318 ****
--- 1313,1321 ----

         mtx_assert(&vm_page_queue_mtx, MA_OWNED);

+       if (pv->pv_flags & PVF_EXEC)
+               return();
+
         /* the cache gets written back/invalidated on context switch.
          * therefore, if a user page shares an entry in the same page or
          * with the kernel map and at least one is writable, then the

--Mark


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