linux-kernel - Re: [RFC patch 7/7] timekeeping: Hack to use fine grained timestamps during boot

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Date:   Tue, 28 Nov 2017 13:43:46 -0500
From:   Prarit Bhargava <prarit@...hat.com>
To:     Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [RFC patch 7/7] timekeeping: Hack to use fine grained timestamps
 during boot



On 11/23/2017 07:58 AM, Petr Mladek wrote:
> On Wed 2017-11-15 19:15:38, Thomas Gleixner wrote:
>> For demonstration purposes only.
>>
>> Add a disgusting hack to work around the fact that high resolution clock
>> MONOTONIC accessors are not available during early boot and return stale
>> time stamps accross suspend/resume when the current clocksource is not
>> flagged with CLOCK_SOURCE_SUSPEND_ACCESS_OK.
>>
>> Use local_clock() to provide timestamps in early boot and when the
>> clocksource is not accessible after timekeeping_suspend(). In the
>> suspend/resume case this might cause non monotonic timestamps.
> 
> I get the non-monotonic times even during boot:
> 
> [    0.026709] smp: Bringing up secondary CPUs ...
> [    0.027973] x86: Booting SMP configuration:
> [    0.028006] .... node  #0, CPUs:      #1
> [    0.004000] kvm-clock: cpu 1, msr 1:3ff51041, secondary cpu clock
>      ^^^^^^^^
> [    0.032097] KVM setup async PF for cpu 1
> [    0.032766] kvm-stealtime: cpu 1, msr 13b00dc40
> [    0.036502]  #2
> [    0.004000] kvm-clock: cpu 2, msr 1:3ff51081, secondary cpu clock
>      ^^^^^^^^
> [    0.040062] KVM setup async PF for cpu 2
> [    0.040576] kvm-stealtime: cpu 2, msr 13b20dc40
> [    0.041304]  #3
> [    0.004000] kvm-clock: cpu 3, msr 1:3ff510c1, secondary cpu clock
>      ^^^^^^^^
> [    0.048051] KVM setup async PF for cpu 3
> [    0.048554] kvm-stealtime: cpu 3, msr 13b40dc40
> 
> 
> To be honest, I do not feel experienced enough to decide which
> solution is acceptable. I would say that only few people care
> about timestamps during boot. On the other hand, some tools

It is extremely important to know what happened and how long it took.  I agree
with Petr, we should figure out a way to guarantee that the timestamp is monotonic.

P.