linux-kernel - RE: Direct rdtsc call side-effect

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Message-ID: <353732783fde46919fdcf698c326b7ed@AcuMS.aculab.com>
Date:   Mon, 5 Jun 2023 10:27:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>
CC:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Steven Noonan <steven@...inklabs.net>,
        "kernel@...labora.com" <kernel@...labora.com>
Subject: RE: Direct rdtsc call side-effect

...
> Who would have thought that rdtsc() in applications can be a problem.
> Interfaces to query time exist for a reason and it's documented by
> Microsoft:
> 
> https://learn.microsoft.com/en-us/windows/win32/dxtecharts/game-timing-and-multicore-processors
> 
> But sure, reading documentation is overrated...

That eve says:
   "Multiprocessor and dual-core systems do not guarantee synchronization
   of their cycle counters between cores."

.
> Synchronizing TSC by writing the TSC MSR is fragile as hell. This has
> been tried so often and never reliably passed all synchronization tests
> on a wide range of systems.
> 
> It kinda works on single socket, but not on larger systems.
> 
> We spent an insane amount of time to make timekeeping correct and I'm
> not interested at all to deal with the fallout of such a mechanim.

I've wondered whether the TSC ought to be deliberately mis-synchronised?
So the high order bits are effectively the cpu number.

It has to be said that using it as a time source was fundamentally
a bad idea.
Sometimes (eg micro benchmarks) you really want a TSC.
You can extract one from the performance counters, but it is hard,
root only, and the library functions have high and variable overhead.

	David

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