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

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Message-Id: <CFEB6DDC-D0C6-4B0C-9679-CB743BDCE3FC@gmail.com>
Date:   Wed, 31 Jan 2018 12:52:29 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...capital.net>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86: Align TLB invalidation info

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> On 01/31/2018 12:48 PM, Nadav Amit wrote:
>>>> On Jan 31, 2018, at 12:11 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.
>>>> 
>>>> GCC provides a way to deal with it by using
>>>> __builtin_alloca_with_align(). Use it to avoid the bouncing cache lines.
>>> Eww.  How about __aligned?
>> Err.. Stupid me. For some reason I remembered I tried it and it didn’t have
>> the desired effect, which caused me to assume it does not work for variables
>> on the stack. Anyhow, it does the work. I’ll submit v2.
> 
> Also, just curious, but did you find this situation by inspection or did
> it show up in a profile somewhere?

Actually, not. I considered adding data to info for a different reason
(which eventually I deserted); I was assuming you will all kill me for
increasing the size, so this was a preemption step.