On Thu, 12 Apr 2007 11:36:02 +0200 Andi Kleen <[email protected]> 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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