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

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Message-Id: <95FB622E-4E2B-4EFC-998F-4A3522BA27BD@gmail.com>
Date:   Thu, 1 Feb 2018 10:45:55 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] x86: Align TLB invalidation info

Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Jan 31, 2018 at 09:38:46PM -0800, Nadav Amit wrote:
> 
>> 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.
> 
> 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.
> 
> A microbench and/or perf is what you should use for this.

Don’t 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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