* Denys Vlasenko <[email protected]> wrote:
> This mimics the recent similar 64-bit change.
> Saves ~110 bytes of code.
>
> Patch was run-tested on 32 and 64 bits, Intel and AMD CPU.
> I also looked at the diff of entry_64.o disassembly, to have
> a different view of the changes.
The other important question would be: what performance difference (if
any) did you observe before/after the change?
Thanks,
Ingo
On 04/01/2015 10:51 AM, Ingo Molnar wrote:
>
> * Denys Vlasenko <[email protected]> wrote:
>
>> This mimics the recent similar 64-bit change.
>> Saves ~110 bytes of code.
>>
>> Patch was run-tested on 32 and 64 bits, Intel and AMD CPU.
>> I also looked at the diff of entry_64.o disassembly, to have
>> a different view of the changes.
>
> The other important question would be: what performance difference (if
> any) did you observe before/after the change?
I did not measure it then.
At the moment I don't have AMD CPUs here, cant benchmark
32-bit syscall-based codepath.
On a Sandy Bridge CPU (IOW: sysenter codepath) -
Before: 78.57 ns per getpid
After: 76.90 ns per getpid
It's better than I thought it would be.
Probably because this load:
movl ASM_THREAD_INFO(TI_sysenter_return, %rsp, 0), %r10d
has been moved up by the patch (happens sooner).
* Denys Vlasenko <[email protected]> wrote:
> On 04/01/2015 10:51 AM, Ingo Molnar wrote:
> >
> > * Denys Vlasenko <[email protected]> wrote:
> >
> >> This mimics the recent similar 64-bit change.
> >> Saves ~110 bytes of code.
> >>
> >> Patch was run-tested on 32 and 64 bits, Intel and AMD CPU.
> >> I also looked at the diff of entry_64.o disassembly, to have
> >> a different view of the changes.
> >
> > The other important question would be: what performance difference (if
> > any) did you observe before/after the change?
>
> I did not measure it then.
>
> At the moment I don't have AMD CPUs here, cant benchmark
> 32-bit syscall-based codepath.
>
> On a Sandy Bridge CPU (IOW: sysenter codepath) -
>
> Before: 78.57 ns per getpid
> After: 76.90 ns per getpid
>
> It's better than I thought it would be.
> Probably because this load:
>
> movl ASM_THREAD_INFO(TI_sysenter_return, %rsp, 0), %r10d
>
> has been moved up by the patch (happens sooner).
There's also less I$ used, and in straight, continuous spots, which
should result in less cache misses in the very common "the kernel's
code is cache cold" situation that syscall entry operates under - and
that's not captured by your benchmark.
So it's a good change.
Thanks,
Ingo
On Wed, Apr 01, 2015 at 03:12:50PM +0200, Denys Vlasenko wrote:
> At the moment I don't have AMD CPUs here, cant benchmark
> 32-bit syscall-based codepath.
You could send me your measuring tool - I'll run it on AMD.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
--