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Subject: Re: [PATCH] x86: Align TLB invalidation info
To: Nadav Amit <nadav.amit@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>
References: <20180131201118.1694-1-namit@vmware.com>
 <BD28D2CF-CAD7-45A1-87D0-DE38F7B0EFE5@amacapital.net>
 <747E60F0-2315-4BEC-895E-0E29C57A0AF0@gmail.com>
Cc: 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>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <1e0c2666-d0ee-8fe2-3272-6b49bf045834@linux.intel.com>
Date: Wed, 31 Jan 2018 12:49:47 -0800
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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?