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SubjectRe: [PATCH 2/9] x86/asm/entry/32: Use PUSH instructions to build pt_regs on stack

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> On 04/01/2015 10:51 AM, Ingo Molnar wrote:
> >
> > * Denys Vlasenko <dvlasenk@redhat.com> 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


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