* Denys Vlasenko <dvlas...@redhat.com> wrote:
> On 05/13/2015 12:17 PM, Ingo Molnar wrote:
> >>> In any case, the interesting measurement would not be -Os comparisons
> >>> (which causes GCC to be too crazy), but to see the size effect of your
> >>> _patch_ that always-inlines spinlock ops, on plain defconfig and on
> >>> defconfig-Os.
> >>
> >> Here it is:
> >>
> >> text data bss dec hex filename
> >> 12335864 1746152 1081344 15163360 e75fe0 vmlinuxO2.before
> >> 12335930 1746152 1081344 15163426 e76022 vmlinux
> >
> > Hm, that's a (small) size increase on O2.
> >
> > That might be a net positive though: because now we've eliminated
> > quite a few function calls. Do we know which individual functions
> > bloat and which debloat?
>
> >> text data bss dec hex filename
> >> 10373764 1684200 1077248 13135212 c86d6c vmlinuxOs.before
> >> 10363621 1684200 1077248 13125069 c845cd vmlinux
> >
> > A decrease - which gets exploded on allyesconfig.
> >
> > So as long as the -O2 case does not get hurt we can do -Os fixes.
> >
> > I think this needs a bit more work to ensure that the O2 case is a
> > net win.
>
> I think O2 difference is just noise: with -O2 gcc is far less prone
> to bogus deinlining, my patch should have negligible effect. And
> effect is indeed negligible: +70 bytes on 12 megabytes.
So the patch force-inlines about a dozen locking APIs:
- Some of those decrease the defconfig kernel size.
Which ones and by how much?
- Some of those increase the defconfig kernel size.
Which ones and by how much?
We only know that the net effect is +70 bytes. Does that come out of:
- large fluctuations such as -1000-1000+1000+1070, which happens to
net out into a small net number?
- or does it come from much smaller fluctuations?
So to make an informed decision we need to know those details. When I
deinline or reinline functions I usually do it on a per function
basis, to avoid such ambiguity.
In the end what we want to have is only those deinlining/reinlining
changes that decrease the defconfig kernel size, or at worst only
increase it marginally.
Thanks,
Ingo
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