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From:   Nadav Amit <nadav.amit@gmail.com>
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Subject: Re: [PATCH] x86: Align TLB invalidation info
Date:   Thu, 1 Feb 2018 10:45:55 -0800
In-Reply-To: <20180201093847.GZ2269@hirez.programming.kicks-ass.net>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
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Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Jan 31, 2018 at 09:38:46PM -0800, Nadav Amit wrote:
>=20
>> 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.
>=20
> However cool ftrace is, it is _really_ bad for such uses. The cost of
> using ftrace is many many time higher than any change you could affect
> by this.
>=20
> A microbench and/or perf is what you should use for this.

Don=E2=80=99t expect to see a remote NUMA access impact, whose cost are =
few 10s of
nanoseconds on microbenchmarks. (And indeed I did not.) Each iteration =
of
#PF - MADV_DONTNEED takes several microseconds, and the impact is lost =
in
the noise.

You are right in the fact that ftrace introduces overheads, but the =
variance
is relatively low. If I stretch the struct to 3 lines of cache, I see a =
20ns
overhead. Anyhow, I think this line of code got more than its fair share =
of
attention.


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