On 07/22/2010 03:25 PM, Benjamin Herrenschmidt wrote:
> On Thu, 2010-07-22 at 11:24 -0700, Darren Hart wrote:
>>
>> 1) How can the preempt_count() get mangled across the H_CEDE hcall?
>> 2) Should we call preempt_enable() in cpu_idle() prior to cpu_die() ?
>
> The preempt count is on the thread info at the bottom of the stack.
>
> Can you check the stack pointers ?
Hi Ben, thanks for looking.
I instrumented the area around extended_cede_processor() as follows
(please confirm I'm getting the stack pointer correctly).
while (get_preferred_offline_state(cpu) == CPU_STATE_INACTIVE) {
asm("mr %0,1" : "=r" (sp));
printk("before H_CEDE current->stack: %lx, pcnt: %x\n", sp,
preempt_count());
extended_cede_processor(cede_latency_hint);
asm("mr %0,1" : "=r" (sp));
printk("after H_CEDE current->stack: %lx, pcnt: %x\n", sp,
preempt_count());
}
On Mainline (2.6.33.6, CONFIG_PREEMPT=y) I see this:
Jul 22 18:37:08 igoort1 kernel: before H_CEDE current->stack: c00000010e9e3ce0,
pcnt: 1
Jul 22 18:37:08 igoort1 kernel: after H_CEDE current->stack: c00000010e9e3ce0,
pcnt: 1
This surprised me as preempt_count is 1 before and after, so no
corruption appears to occur on mainline. This makes the pcnt of 65 I see
without the preempt_count()=0 hack very strange. I ran several hundred
off/on cycles. The issue of preempt_count being 1 is still addressed by
this patch however.
On PREEMPT_RT (2.6.33.5-rt23 - tglx, sorry, rt/2.6.33 next time, promise):
Jul 22 18:51:11 igoort1 kernel: before H_CEDE current->stack: c000000089bcfcf0,
pcnt: 1
Jul 22 18:51:11 igoort1 kernel: after H_CEDE current->stack: c000000089bcfcf0,
pcnt: ffffffff
In both cases the stack pointer appears unchanged.
Note: there is a BUG triggered in between these statements as the
preempt_count causes the printk to trigger:
Badness at kernel/sched.c:5572
Thanks,
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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