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From: Petr Mladek <pmladek@suse.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linuxfoundation.org>,
	Prarit Bhargava <prarit@redhat.com>,
	Mark Salyzyn <salyzyn@android.com>,
	Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joe Perches <joe@perches.com>
Subject: Re: [RFC patch 7/7] timekeeping: Hack to use fine grained timestamps during boot
Date: Thu, 23 Nov 2017 13:58:23 +0100	[thread overview]
Message-ID: <20171123125823.gnhwtnx6bxd3tb4q@pathway.suse.cz> (raw)
In-Reply-To: <20171115182657.703928462@linutronix.de>

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
might get confused by non-monotonic timestamps. Therefore
I would try to avoid this patch.

But I might be completely wrong. After all, this feature is
done because people wanted to track the time more precisely.
Some of them wanted to have the most precise time even during
suspend and resume. On the other hand, it might be enough
if we allow to get the information from the other
stored timestamps.

Best Regards,
Petr

  reply	other threads:[~2017-11-23 12:58 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-15 18:15 [RFC patch 0/7] printk: Switch to CLOCK_MONOTONIC and store extra time stamps Thomas Gleixner
2017-11-15 18:15 ` [RFC patch 1/7] timekeeping: Do not unconditionally suspend NMI safe timekeepers Thomas Gleixner
2017-11-15 18:15 ` [RFC patch 2/7] x86/tsc: Set clocksource CLOCK_SOURCE_SUSPEND_ACCESS_OK Thomas Gleixner
2017-11-15 18:15 ` [RFC patch 3/7] printk: Use clock MONOTONIC for timestamps Thomas Gleixner
2017-11-16  7:58   ` Sergey Senozhatsky
2017-11-15 18:15 ` [RFC patch 4/7] timekeeping: Add NMI safe accessor to mono/boot/real clocks Thomas Gleixner
2017-11-17 23:00   ` Steven Rostedt
2017-11-17 23:12     ` Linus Torvalds
2017-11-17 23:43       ` Thomas Gleixner
2017-11-15 18:15 ` [RFC patch 5/7] crash: Add VMCOREINFO_FIELD_AND_OFFSET() Thomas Gleixner
2017-11-23 12:46   ` Petr Mladek
2017-11-15 18:15 ` [RFC patch 6/7] printk: Store mono/boot/real time timestamps Thomas Gleixner
2017-11-23 13:36   ` Petr Mladek
2017-11-15 18:15 ` [RFC patch 7/7] timekeeping: Hack to use fine grained timestamps during boot Thomas Gleixner
2017-11-23 12:58   ` Petr Mladek [this message]
2017-11-28 18:43     ` Prarit Bhargava
2017-11-28 18:47       ` Thomas Gleixner
2017-12-08 11:23         ` Petr Mladek
2017-12-08 19:51           ` Thomas Gleixner
2017-11-28 19:10       ` Mark Salyzyn
2017-11-28 19:45         ` Steven Rostedt
2017-11-28 20:29           ` Mark Salyzyn
2017-11-28 20:38             ` Peter Zijlstra

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