Re: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections

Kees Cook
2020-08-21T19:18:18Z

On Tue, Aug 04, 2020 at 12:06:49PM -0400, Arvind Sankar wrote:
> On Mon, Aug 03, 2020 at 09:45:32PM -0700, Andi Kleen wrote:
> > > Why is that? Both .text and .text.hot have alignment of 2^4 (default
> > > function alignment on x86) by default, so it doesn't seem like it should
> > > matter for packing density. Avoiding interspersing cold text among
> >
> > You may lose part of a cache line on each unit boundary. Linux has
> > a lot of units, some of them small. All these bytes add up.
>
> Separating out .text.unlikely, which isn't aligned, slightly _reduces_
> this loss, but not by much -- just over 1K on a defconfig. More
> importantly, it moves cold code out of line (~320k on a defconfig),
> giving better code density for the hot code.
>
> For .text and .text.hot, you lose the alignment padding on every
> function boundary, not unit boundary, because of the 16-byte alignment.
> Whether .text.hot and .text are arranged by translation unit or not
> makes no difference.
>
> With *(.text.hot) *(.text) you get HHTT, with *(.text.hot .text) you get
> HTHT, but in both cases the individual chunks are already aligned to 16
> bytes. If .text.hot _had_ different alignment requirements to .text, the
> HHTT should actually give better packing in general, I think.

Okay, so at the end of the conversation, I think it looks like this
patch is correct: it collects the hot, unlikely, etc into their own
areas (e.g. HHTTUU is more correct than HTUHTU), so this patch stands
as-is.

--
Kees Cook