linux-kernel - Re: [PATCH v2] x86: Align TLB invalidation info

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Message-ID: <CALCETrV+4Yri3hr5fSDSj7di+z3ZnBpQWQO7w7vfWVT+N_XrOg@mail.gmail.com>
Date:   Wed, 31 Jan 2018 13:03:24 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2] x86: Align TLB invalidation info

On Wed, Jan 31, 2018 at 1:00 PM, Nadav Amit <namit@...are.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@...are.com>
>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>

Acked-by: Andy Lutomirski <luto@...nel.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
>