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

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Message-ID: <4959DDEE-58DC-45E7-BE00-019A45A97D2B@zytor.com>
Date:   Tue, 16 May 2023 10:57:13 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Dave Hansen <dave.hansen@...el.com>, Rong Tao <rtoax@...mail.com>,
        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>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/vdso: Use non-serializing instruction rdtsc

On May 16, 2023 7:12:34 AM PDT, Dave Hansen <dave.hansen@...el.com> wrote:
>On 5/15/23 23:52, Rong Tao wrote:
>> Replacing rdtscp or 'lfence;rdtsc' with the non-serializable instruction
>> rdtsc can achieve a 40% performance improvement with only a small loss of
>> precision.
>
>I think the minimum that can be done in a changelog like this is to
>figure out _why_ a RDTSCP was in use.  There are a ton of things that
>can make the kernel go faster, but not all of them are a good idea.
>
>I assume that the folks that wrote this had good reason for not using
>plain RSTSC.  What were those reasons?

I believe the motivation is that it is atomic with reading the CPU number.