linux-kernel - Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early in boot

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Message-ID: <1dd60ace-c2aa-6c88-d4b0-cba934be4b79@cn.fujitsu.com>
Date:   Thu, 28 Sep 2017 18:03:05 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Pasha Tatashin <pasha.tatashin@...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>,
        <tglx@...utronix.de>, <hpa@...or.com>
Subject: Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early
 in boot


Hi Peter,

At 09/28/2017 02:09 AM, Peter Zijlstra wrote:
> On Wed, Sep 27, 2017 at 08:05:48PM +0200, Peter Zijlstra wrote:
>> On Wed, Sep 27, 2017 at 09:52:36PM +0800, Dou Liyang wrote:
>>> We do not want to do that. Because, we use "notsc" to support Dynamic
>>> Reconfiguration[1].
>>>
>>> AFAIK, this feature enables hot-add system board which contains CPUs
>>> and memories. But the CPUs in different board may have different TSCs
>>> which are not consistent with the TSC from the existing CPUs. If we hot-add
>>> a board directly, the machine may happen the inconsistency of
>>> TSC.
>>>
>>> We make our effort to specify the same TSC value as existing one through
>>> hardware and firmware, but it is hard. So we recommend to specify
>>> "notsc" option in command line for users who want to use Dynamic
>>> Reconfiguration.
>>
>> Oh gawd, that's horrific. And in my book a good reason to kill that
>> option.
>
> That is, even with unsynchronized TSC we're better off using RDTSC. The
> whole mess in kernel/sched/clock.c is all about getting semi sensible
> results out of unsynchronized TSC.
>

It will be best if we can support TSC sync capability in x86, but seems
is not easy.

Thanks,

	dou.

> There really is no reason to artificially kill TSC usage.
>
>
>