linux-kernel - Re: [PATCH linux-next v3] x86/xen/time: prefer tsc as clocksource when it is invariant

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Date:   Mon, 13 Feb 2023 09:18:20 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Krister Johansen <kjlx@...pleofstupid.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jan Beulich <jbeulich@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Anthony Liguori <aliguori@...zon.com>,
        David Reaver <me@...idreaver.com>,
        Brendan Gregg <brendan@...el.com>
Subject: Re: [PATCH linux-next v3] x86/xen/time: prefer tsc as clocksource
 when it is invariant

On 16.12.22 17:21, Krister Johansen wrote:
> Kvm elects to use tsc instead of kvm-clock when it can detect that the
> TSC is invariant.
> 
> (As of commit 7539b174aef4 ("x86: kvmguest: use TSC clocksource if
> invariant TSC is exposed")).
> 
> Notable cloud vendors[1] and performance engineers[2] recommend that Xen
> users preferentially select tsc over xen-clocksource due the performance
> penalty incurred by the latter.  These articles are persuasive and
> tailored to specific use cases.  In order to understand the tradeoffs
> around this choice more fully, this author had to reference the
> documented[3] complexities around the Xen configuration, as well as the
> kernel's clocksource selection algorithm.  Many users may not attempt
> this to correctly configure the right clock source in their guest.
> 
> The approach taken in the kvm-clock module spares users this confusion,
> where possible.
> 
> Both the Intel SDM[4] and the Xen tsc documentation explain that marking
> a tsc as invariant means that it should be considered stable by the OS
> and is elibile to be used as a wall clock source.
> 
> In order to obtain better out-of-the-box performance, and reduce the
> need for user tuning, follow kvm's approach and decrease the xen clock
> rating so that tsc is preferable, if it is invariant, stable, and the
> tsc will never be emulated.
> 
> [1] https://aws.amazon.com/premiumsupport/knowledge-center/manage-ec2-linux-clock-source/
> [2] https://www.brendangregg.com/blog/2021-09-26/the-speed-of-time.html
> [3] https://xenbits.xen.org/docs/unstable/man/xen-tscmode.7.html
> [4] Intel 64 and IA-32 Architectures Sofware Developer's Manual Volume
>      3b: System Programming Guide, Part 2, Section 17.17.1, Invariant TSC
> 
> Signed-off-by: Krister Johansen <kjlx@...pleofstupid.com>
> Code-reviewed-by: David Reaver <me@...idreaver.com>

Pushed to: xen/tip.git for-linus-6.3


Juergen


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