Subject: Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries

From: Darren Hart <[email-masked]>
Date: Thu, 05 Aug 2010 19:19:00 -0700

On 07/22/2010 10:09 PM, Benjamin Herrenschmidt wrote:
> On Thu, 2010-07-22 at 21:44 -0700, Darren Hart wrote:
> 
>>   suggestion I updated the instrumentation to display the
>> local_save_flags and irqs_disabled_flags:
> 
>> Jul 22 23:36:58 igoort1 kernel: local flags: 0, irqs disabled: 1
>> Jul 22 23:36:58 igoort1 kernel: before H_CEDE current->stack: c00000010e9e3ce0, pcnt: 1
>> Jul 22 23:36:58 igoort1 kernel: after H_CEDE current->stack: c00000010e9e3ce0, pcnt: 1
>>
>> I'm not sure if I'm reading that right, but I believe interrupts are
>> intended to be disabled here. If accomplished via the
>> spin_lock_irqsave() this would behave differently on RT. However, this
>> path disables the interrupts handled by xics, all but the IPIs anyway.
>> On RT I disabled the decrementer as well.
>>
>> Is it possible for RT to be receiving other interrupts here?
> 
> Also you may want to call hard_irq_disable() to -really- disable
> interrupts ... since we do lazy-disable on powerpc
> 

A quick update and request for direction wrt the cede hcall, interrupt handling, and the stack pointer.

I've added patches one at a time, eliminating bugs with preempt_rt
(tip/rt/head). With my current patchset I have no crashes with a single run of
random_online.sh (stress testing to hit the hang will sees is my todo for
tomorrow).

Current patchset includes:
patches/0001-wms-fix01.patch # P7 lazy flushing thing

# next four are sent to / queued for mainline
patches/powerpc-increase-pseries_cpu_die-delay.patch
patches/powerpc-enable-preemption-before-cpu_die.patch
patches/powerpc-silence-__cpu_up-under-normal-operation.patch
patches/powerpc-silence-xics_migrate_irqs_away-during-cpu-offline.patch

# this one needs to be cleaned up and sent to mainline
patches/powerpc-wait-for-cpu-to-go-inactive.patch

patches/powerpc-disable-decrementer-on-offline.patch
patches/powerpc-cpu_die-preempt-hack.patch # reset preempt_count to 0 after cede
patches/powerpc-cede-processor-inst.patch # instrumentation
patches/powerpc-hard_irq_disable.patch # hard_irq_disable before cede
patches/powerpc-pad-thread_info.patch

I didn't include all the patches as the relevant bits are included below in
code form.

With the instrumentation, it's clear the change to preempt_count() is targeted
(since I added padding before and after preempt_count in the thread_info
struct) and preempt_count is still changed. It is also clear that it isn't just
a stray decrement since I set preempt_count() to 4 prior to calling cede and it
still is 0xffffffff after the hcall. Also note that the stack pointer doesn't
change across the cede call and neither does any other part of the thread_info
structure.

Adding hard_irq_disable() did seem to affect things somewhat. Rather than a
serialized list of before/after thread_info prints around cede, I see
several befores then several afters. But the preempt_count is still modified.

The relevant code looks like this (from pseries_mach_cpu_die())

                        hard_irq_disable(); /* this doesn't fix things... but
			                       does change behavior a bit */
                        preempt_count() = 0x4; 
                        asm("mr %0,1" : "=r" (sp));  /* stack pointer is in R1 */
                        printk("before cede: sp=%lx pcnt=%x\n", sp, preempt_count()); 
                        print_thread_info(); 
                        extended_cede_processor(cede_latency_hint); 
                        asm("mr %0,1" : "=r" (sp)); 
                        printk("after cede: sp=%lx pcnt=%x\n", sp, preempt_count()); 
                        print_thread_info(); 
                        preempt_count() = 0; 

With these patches applied, the output across cede looks like:

before cede: sp=c000000000b57150 pcnt=4
*** current->thread_info ***
ti->task: c000000000aa1410
ti->exec_domain: c000000000a59958
ti->cpu: 0
ti->stomp_on_me: 57005 /* 0xDEAD - forgot to print in hex */
ti->preempt_count: 4
ti->stomp_on_me_too: 48879 /* 0xBEEF - forgot to print in hex */
ti->local_flags: 0
*** end thread_info ***
after cede: sp=c000000000b57150 pcnt=ffffffff
*** current->thread_info ***
ti->task: c000000000aa1410
ti->exec_domain: c000000000a59958
ti->cpu: 0
ti->stomp_on_me: 57005
ti->preempt_count: ffffffff
ti->stomp_on_me_too: 48879
ti->local_flags: 0
*** end thread_info ***

Are there any additional thoughts on what might be causing preempt_count to change?
I was thinking that cede might somehow force it to 0 (or perhaps one of the
preempt_count explicit value setters in irq.c) and then decrement it - but that dec
should trigger an error in the dec_preempt_count() as I have CONFIG_DEBUG_PREEMPT=y.

...

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team