linux-kernel - Re: [PATCH] x86/vdso: Use non-serializing instruction rdtsc

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Date:   Tue, 16 May 2023 14:53:24 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Rong Tao" <rtoax@...mail.com>,
        "Thomas Gleixner" <tglx@...utronix.de>
Cc:     "Rong Tao" <rongtao@...tc.cn>, "Ingo Molnar" <mingo@...hat.com>,
        "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/vdso: Use non-serializing instruction rdtsc

On Mon, May 15, 2023, at 11:52 PM, Rong Tao wrote:
> From: Rong Tao <rongtao@...tc.cn>
>
> Replacing rdtscp or 'lfence;rdtsc' with the non-serializable instruction
> rdtsc can achieve a 40% performance improvement with only a small loss of
> precision.
>
> The RDTSCP instruction is not a serializing instruction, but it does wait
> until all previous instructions have executed and all previous loads are
> globally visible. The RDTSC instruction is not a serializing instruction.
> It does not necessarily wait until all previous instructions have been
> executed before reading the counter.
>
> Record the time-consuming of vdso clock_gettime(), pseudo code:
>
>     count = 1000 * 1000 * 100;
>     while (count--)
>         clock_gettime(CLOCK_REALTIME, &ts);
>
> Time-consuming comparison:
>
>      Time Consume(ns) | rdtsc_ordered() |  rdtsc()  | Promote
>     ------------------+-----------------+-----------+---------
>     Physical Machine  |  1269147289     | 759067324 |   40%
>      Guest OS (KVM)   |  1756615963     | 995823886 |   43%
>
> Signed-off-by: Rong Tao <rongtao@...tc.cn>

Out of curiosity, what happens if you apply that patch and run this thing:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/misc-tests.git/tree/evil-clock-test.cc

Build it with g++ -O2 and run:

./evil-clock-test -c monotonic

--Andy