Weird sched_clock behaviour during boot with -rc1
John Stultz
john.stultz at linaro.org
Tue Feb 4 15:46:03 EST 2014
On 02/04/2014 10:36 AM, Will Deacon wrote:
> Hi guys,
>
> Booting -rc1 on my TC2 gives the following strange entries in the dmesg:
>
>
> Uncompressing Linux... done, booting the kernel.
> [ 0.000000] Booting Linux on physical CPU 0x0
>
> [...]
>
> [ 0.000000] HighMem zone: 329728 pages, LIFO batch:31
> [ 7.789662] sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 178956969942ns
> [ 0.000129] PERCPU: Embedded 9 pages/cpu @ee7bd000 s12800 r8192 d15872 u36864
>
> [...]
>
> [ 0.868297] NR_IRQS:16 nr_irqs:16 16
> [ 0.886350] Architected cp15 timer(s) running at 24.00MHz (phys).
> [ 2915.164998] sched_clock: 56 bits at 24MHz, resolution 41ns, wraps every 2863311519744ns
> [ 0.000002] Switching to timer-based delay loop
> [ 0.014249] Console: colour dummy device 80x30
>
>
> so it looks like something whacky goes on during sched_clock registration.
> Sure enough, we're doing a pr_info in-between updating cs.* and calling
> update_sched_clock(), so moving the print sorts things out (diff below).
Yea... we have to be particularly careful with sched_clock to avoid
locks since we don't want to deadlock, but in this case
sched_clock_register is a little too relaxed here.
Stephen: Would it make sense to set cd.suspended = true at the top of
the registration? That should block any sched_clock calls from getting
half-updated data, but still allow the sched_clock_update function to work.
> What I can't figure out is why this has suddenly started happening with
> 3.14. Any ideas?
No clue. I'm guessing something like timing changes in the printk paths
that call sched_clock?
I suspect your patch to move the print down will also be a good idea
along with the suspending sched_clock while we register new ones.
thanks
-john
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