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Subject: Re: [PATCH] x86: Align TLB invalidation info
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <f7270e0d-d1d1-599d-d2c9-ddc2c263e090@linux.intel.com>
Date:   Wed, 31 Jan 2018 21:38:46 -0800
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>
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References: <20180131201118.1694-1-namit@vmware.com>
 <8bb352bc-4e1f-4e87-80e3-a8e65d618d2a@linux.intel.com>
 <0E65629C-0D7D-4602-A43D-B18E62F330A6@gmail.com>
 <f7270e0d-d1d1-599d-d2c9-ddc2c263e090@linux.intel.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 01/31/2018 01:09 PM, Nadav Amit wrote:
>>> 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.
>=20
> I'm certainly not nak'ing this.  I think your patch is likely a good
> idea.  But, could you please take ten or twenty minutes to go see if
> practice matches your assumptions?  I'd really appreciate it.  If you
> can't measure it, then no biggie.

[CC=E2=80=99ing the mailing list]

Per your request, I measured it (which perhaps I should have done =
before). I
caused a misalignment intentionally by adding some padding to =
flush_tlb_info
and compared it with an aligned version.

I used ftrace to measure the execution time of flush_tlb_func_remote() =
on a
2-socket Haswell machine, using a microbenchmark I wrote for some =
research
project.

It turns out that your skepticism may be correct - In both cases the
function execution time is roughly 400ns (2% improvement on the aligned =
case
which is probably noise).

So it is up to you whether you want to discard the patch.

Regards,
Nadav=