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
