linux-kernel - Re: [PATCH v12 03/11] x86/tsc: redefine notsc to behave as tsc=unstable

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Date:   Sat, 23 Jun 2018 15:32:36 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
        peterz@...radead.org, prarit@...hat.com, feng.tang@...el.com,
        pmladek@...e.com, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v12 03/11] x86/tsc: redefine notsc to behave as
 tsc=unstable

On Thu, 21 Jun 2018, Pavel Tatashin wrote:
> Currently, notsc kernel parameter disables the use of tsc register by
> sched_clock(). However, this parameter does not prevent linux from
> accessing tsc in other places in kernel.
> 
> The only rational to boot with notsc is to avoid timing discrepancies on
> multi-socket systems where different tsc frequencies may present, and thus
> fallback to jiffies for clock source.
> 
> However, there is another method to solve the above problem, it is to boot
> with tsc=unstable parameter. This parameter allows sched_clock() to use tsc
> but in case tsc is outside of expected interval it is corrected back to a
> sane value.
> 
> This is why there is no reason to keep notsc, and it can be removed. But,
> for compatibility reasons we will keep this parameter but change its
> definition to be the same as tsc=unstable.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
> Reviewed-by: Dou Liyang <douly.fnst@...fujitsu.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>