On 9/24/15, Michael Ellerman <mich...@ellerman.id.au> wrote:
>
>
> On 23 September 2015 16:05:02 GMT+10:00, Michael Neuling <mi...@neuling.org>
> wrote:
>>The 32 and 64 bit variants of __get_datapage() use a "bcl; mflr" to
>>determine the loaded address of the VDSO. The current version of these
>>attempt to use the special bcl variant which avoids pushing to the
>>link stack.
>>
>>Unfortunately it uses bcl+8 rather than the required bcl+4. Hence the
>>current code results in link stack corruption and the resulting
>>performance degradation (due to branch mis-prediction).
>>
>>This patch moves us to bcl+4 by moving __kernel_datapage_offset
>>out of __get_datapage().
>>
>>With this patch, running the below benchmark we get a bump in
>>performance on POWER8 for gettimeofday() (which uses
>>__get_datapage()).
>>
>>64bit gets ~4% improvement:
>> Without patch:
>> # ./tb
>> time = 0.180321
>> With patch:
>> # ./tb
>> time = 0.187408
>>
>>32bit gets ~9% improvement:
>> Without patch:
>> # ./tb
>> time = 0.276551
>> With patch:
>> # ./tb
>> time = 0.252767
>>
>>Testcase tb.c (stolen from Anton)
>> /* gcc -O2 tb.c -o tb */
>> #include <sys/time.h>
>> #include <stdio.h>
>>
>> int main()
>> {
>> int i;
>>
>> struct timeval tv_start, tv_end;
>>
>> gettimeofday(&tv_start, NULL);
>>
>> for(i = 0; i < 10000000; i++) {
>> gettimeofday(&tv_end, NULL);
>> }
>>
>> printf("time = %.6f\n", tv_end.tv_sec - tv_start.tv_sec +
>>(tv_end.tv_usec - tv_start.tv_usec) * 1e-6);
>>
>> return 0;
>> }
>
> You know where test cases are supposed to go.
>
> I know it's not a pass/fail test, but it's still useful. If it's in the tree
> it will get run as part of automated test runs and we will have a record of
> the result over time.
I can send a patch for it.
>
> cheers
> --
> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
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