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

["Andy Lutomirski" <luto@kernel.org>]

On Mon, May 15, 2023, at 11:52 PM, Rong Tao wrote:
> From: Rong Tao <rongtao@cestc.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@cestc.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