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

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Message-ID: <8bb352bc-4e1f-4e87-80e3-a8e65d618d2a@linux.intel.com>
Date:   Wed, 31 Jan 2018 13:01:24 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Nadav Amit <namit@...are.com>, x86@...nel.org
Cc:     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>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86: Align TLB invalidation info

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.