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

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Message-ID: <CALCETrX-zmhoLgZzL_VJZqZF+XTS26o+HHA7=vA+E7T88yO_uA@mail.gmail.com>
Date:   Wed, 31 Jan 2018 13:15:19 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <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>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86: Align TLB invalidation info

On Wed, Jan 31, 2018 at 1:09 PM, Nadav Amit <nadav.amit@...il.com> wrote:
> Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>
>> On 01/31/2018 12:11 PM, Nadav Amit 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.
>>
>> It doesn't really *bounce*, though, does it?  I don't see any writes on
>> the remote side.  The remote use seems entirely read-only.
>>
>> You also don't have to exhaustively test this, but I'd love to see at
>> least a sanity check with a microbenchmark (or something) that, yes,
>> this does help *something*.  Maybe it makes the remote
>> flush_tlb_func_common() run faster because it's pulling in fewer lines,
>> or maybe you can even detect fewer misses in there.
>
> I agree that with the whole Meltdown/Spectre entry-cost it might not even be
> measurable, at least on small ( < 2 sockets) machines. But I do not think it
> worth profiling. Basically, AFAIK, all the data structures that are used for
> inter-processor communication by the kernel are aligned, and this is an
> exception.
>

This is only going to be measurable at all on NUMA, I suspect.