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

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Date:   Mon, 19 Jun 2023 18:58:24 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        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

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 ?

Thanks

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