[PATCH v2] x86: Align TLB invalidation info

[PATCH v2] x86: Align TLB invalidation info

[Nadav Amit]

The TLB invalidation info is allocated on the stack, which might cause
it to be unaligned. Since this information may be transferred to
different cores for TLB shootdown, this might result in an additional
cache-line bouncing between the cores.

We do not use __cacheline_aligned() since it also defines the section,
which is inappropriate for stack variables.

Signed-off-by: Nadav Amit <namit@vmware.com>

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>

--
v1 -> v2: use __aligned instead of all the mess (Andy)
---
 arch/x86/mm/tlb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 5bfe61a5e8e3..9690112e3a82 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -576,7 +576,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 {
 	int cpu;
 
-	struct flush_tlb_info info = {
+	struct flush_tlb_info info __aligned(SMP_CACHE_BYTES) = {
 		.mm = mm,
 	};
 
-- 
2.14.1

Re: [PATCH v2] x86: Align TLB invalidation info

[Andy Lutomirski]

On Wed, Jan 31, 2018 at 1:00 PM, Nadav Amit <namit@vmware.com> wrote:
> The TLB invalidation info is allocated on the stack, which might cause
> it to be unaligned. Since this information may be transferred to
> different cores for TLB shootdown, this might result in an additional
> cache-line bouncing between the cores.
>
> We do not use __cacheline_aligned() since it also defines the section,
> which is inappropriate for stack variables.
>
> Signed-off-by: Nadav Amit <namit@vmware.com>
>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>

Acked-by: Andy Lutomirski <luto@kernel.org>

This is basically free and adds no mess, so I think it's probably okay
even in the absence that it's a huge win.

But Dave is right, the commit message needs updating.  It will reduce
the number of cachelines that become shared and then get exclusively
owned by the originator from 2 to 1.  This isn't really "bouncing".

>
> --
> v1 -> v2: use __aligned instead of all the mess (Andy)
> ---
>  arch/x86/mm/tlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 5bfe61a5e8e3..9690112e3a82 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -576,7 +576,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>  {
>         int cpu;
>
> -       struct flush_tlb_info info = {
> +       struct flush_tlb_info info __aligned(SMP_CACHE_BYTES) = {
>                 .mm = mm,
>         };
>
> --
> 2.14.1
>