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

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Date:   Fri, 8 Dec 2017 12:23:12 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Prarit Bhargava <prarit@...hat.com>,
        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 Tue 2017-11-28 19:47:09, Thomas Gleixner wrote:
> On Tue, 28 Nov 2017, Prarit Bhargava wrote:
> > 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
> > >      ^^^^^^^^
> 
> Which is interesting as this should happen even w/o those patches. 

Good point. I really see this even without those patches.

I was confused because I saw the following with this patchset
but without this latest patch:

[    0.016000] smp: Bringing up secondary CPUs ...
[    0.016000] x86: Booting SMP configuration:
[    0.020000] .... node  #0, CPUs:      #1
[    0.020000] kvm-clock: cpu 1, msr 1:3ff52041, secondary cpu clock
[    0.024000] KVM setup async PF for cpu 1
[    0.024000] kvm-stealtime: cpu 1, msr 13fc8d940

It is monotonic. The only drawback is that the resolution of the
monotonic clock is low at this stage.

Best Regards,
Petr