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

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Message-Id: <0E65629C-0D7D-4602-A43D-B18E62F330A6@gmail.com>
Date:   Wed, 31 Jan 2018 13:09:22 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     the arch/x86 maintainers <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 12:11 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.