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Subject: Re: [PATCH] x86: Align TLB invalidation info
From: Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <1e0c2666-d0ee-8fe2-3272-6b49bf045834@linux.intel.com>
Date: Wed, 31 Jan 2018 12:52:29 -0800
Cc: Andy Lutomirski <luto@amacapital.net>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>
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To: Dave Hansen <dave.hansen@linux.intel.com>
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Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 01/31/2018 12:48 PM, Nadav Amit wrote:
>>>> On Jan 31, 2018, at 12:11 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.
>>>> 
>>>> 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.