linux-kernel - RE: [PATCH 1/1] clocksource: hyper-v: Rework clocksource and sched clock setup

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Date:   Mon, 19 Jun 2023 17:41:20 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH 1/1] clocksource: hyper-v: Rework clocksource and sched
 clock setup

From: Daniel Lezcano <daniel.lezcano@...aro.org> Sent: Monday, June 19, 2023 9:58 AM
> 
> On 19/06/2023 18:44, Michael Kelley (LINUX) wrote:
> > From: Daniel Lezcano <daniel.lezcano@...aro.org> Sent: Monday, June 19, 2023 9:16 AM
> >>
> >> On 09/06/2023 17:47, Michael Kelley wrote:
> >>> Current code assigns either the Hyper-V TSC page or MSR-based ref counter
> >>> as the sched clock. This may be sub-optimal in two cases. First, if there
> >>> is hardware support to ensure consistent TSC frequency across live
> >>> migrations and Hyper-V is using that support, the raw TSC is a faster
> >>> source of time than the Hyper-V TSC page.  Second, the MSR-based ref
> >>> counter is relatively slow because reads require a trap to the hypervisor.
> >>> As such, it should never be used as the sched clock. The native sched
> >>> clock based on the raw TSC or jiffies is much better.
> >>>
> >>> Rework the sched clock setup so it is set to the TSC page only if
> >>> Hyper-V indicates that the TSC may have inconsistent frequency across
> >>> live migrations. Also, remove the code that sets the sched clock to
> >>> the MSR-based ref counter. In the cases where it is not set, the sched
> >>> clock will then be the native sched clock.
> >>>
> >>> As part of the rework, always enable both the TSC page clocksource and
> >>> the MSR-based ref counter clocksource. Set the ratings so the TSC page
> >>> clocksource is preferred. While the MSR-based ref counter clocksource
> >>> is unlikely to ever be the default, having it available for manual
> >>> selection is convenient for development purposes.
> >>>
> >>> Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> >>
> >> The patch does not apply, does it depend on another patch?
> >
> > It should apply to linux-next.  It depends on two previous patches from
> > Peter Zijlstra in the sched/core branch of tip.  See:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=sched/core&id=9397fa2ea3e7634f61da1ab76b9eb88ba04dfdfc
> >
> > and
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=sched/core&id=e39acc37db34f6688e2c16e958fb1d662c422c81
>
> Yeah, but the branch is tip/timers/core
> 
> Could you respin against it ?
> 

Ah, OK.  Just to confirm, you are saying that this patch would go through the
tip/timers/core branch, which does *not* have Peter's patches.  Resolving the
merge conflict will be done within the tip branches.

Yes, I'll respin for tip/timers/core.

Michael