Re: CONFIG_PRINTK_TIME woes

[Andrew Morton <akpm@osdl.org>]

"Luck, Tony" <tony.luck@intel.com> wrote:
>
> It has been pointed out to me that ia64 doesn't boot
> with CONFIG_PRINTK_TIME=y.  The issue is the call to
> sched_clock() ... which on ia64 accesses some per-cpu
> data to adjust for possible variations in processor
> speed between different cpus.  Since the per-cpu page
> is not set up for the first few printk() calls, we die.
> 
> Is this an issue on any other architecture?  Most versions
> of sched_clock() seem to just scale jiffies into nanoseconds,
> so I guess they don't.  s390, sparc64, x86 and x86_64 all
> have more sophisticated versions but they don't appear to me
> to have limitations on how early they might be called.
> 
> Possible solutions:
> 
> 1) Fix ia64 version of sched_clock() to check whether
> the per-cpu page hase been initialized before using it.
> 
> I don't like this solution as it will make sched_clock()
> slower for its primary uses in kernel/sched.c ... none of
> which can be called before the per-cpu area is initialized.
> 
> 
> 2) Add some test to the printk() path along the lines of:
> 
>   t = (can_call_sched_clock) ? sched_clock() : 0;
> 
> This is somewhat ugly ... perhaps too unpleasant for words in
> that can_call_sched_clock is a per-cpu concept!
> 
> 3) Make the printk() code get the time in some other way.
> 
> Using sched_clock() here seems wrong.  The ia64 version
> has a big fat comment about not comparing the results of
> two calls from different cpus ... but since printk() calls
> can come from any cpu ... it seems likely that a user who
> turns on PRINTK_TIME might subtract timestamps from two
> lines to see how long it was between the messages.  This
> could have significant error on a system that has been up
> for a long time.
> 
> But I don't have a better suggestion from existing code.
> I assume that sched_clock() was picked as it is both fast and
> lockless.

Yes, using sched_clock() there is a bit abusive.

It's not terribly performance-sensitive in printk().

How about we give each arch a printk_clock()?
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