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

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Date:	Thu, 12 Apr 2007 09:23:48 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <ak@...ell.com>
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

On Thu, 12 Apr 2007 11:36:02 +0200 Andi Kleen <ak@...ell.com> wrote:

> 
> > 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?

I didn't change anything.

If we change printk() to do a read_seqretry(xtime_lock) (as your patches
apparently do), then any printk() inside write_seqlock(xtime_lock) will
hang.

> > 
> > 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.  

Bit sad.  printk timestamping was originally implemented as a way of
observing and measuring bootup delays.  It seems pretty popular now and
probably quite a few people like high resolution on it.

> 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) 
> 

I wasn't proposing any change in sched_clock().

I was proposing that i386 and x86_64 be given a new, lockless,
high-resolution printk_clock().  Presently x86 uses the default
printk_clock(), which uses sched_clock().  Presumably copying the
pre-x86_64-mm-sched-clock-share.patch version of sched_clock() into
printk_clock() will suffice.

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