linux-kernel - Re: [PATCH] i386 tsc: remove xtime_lock'ing around cpufreq notifier

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Message-Id: <200704121136.03098.ak@novell.com>
Date:	Thu, 12 Apr 2007 11:36:02 +0200
From:	Andi Kleen <ak@...ell.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Daniel Walker <dwalker@...sta.com>, linux-kernel@...r.kernel.org,
	johnstul@...ibm.com, tglx@...utronix.de
Subject: Re: [PATCH] i386 tsc: remove xtime_lock'ing around cpufreq notifier

> OK, so I resurrected x86_64-mm-sched-clock-share.patch and
> x86_64-mm-sched-clock64.patch.  The x86_64 box hangs on boot when using
> netconsole and printk timestamps too.  Removing "time" from the kernel boot
> command line prevents that.

Ah. But ktime_get shouldn't printk. Or did you change that?

> 
> This explains why the hang only happens with
> x86_64-mm-log-reason-why-tsc-was-marked-unstable.patch applied, too: that
> patch must be triggering a printk inside xtime_lock.
> 
> Does someone want to cook up a lockless printk_clock() for i386 and x86_64?

Just use jiffies directly in printk. That's only HZ accurate, but should
be good enough for printk.  

One could use pure monotonic xtime as fallback instead of ktime_get in sched_clock. 
The trouble is  just that they might cause sched_clock to go backwards during 
a temporary instability period (cpufreq change) because the xtime will be 
always a bit behind the TSC and a TSC->xtime conversion will lose time.
At least the scheduler doesn't handle backwards time warp on a CPU gratefully.
Ok I guess it could  return max(last_value_before_instability, xtime) 

-Andi

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