From: Darren Hart <dvhltc@us.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
Michael Neuling <mikey@neuling.org>,
Gautham R Shenoy <ego@in.ibm.com>,
Josh Triplett <josh@joshtriplett.org>,
linuxppc-dev@ozlabs.org, Will Schmidt <willschm@us.ibm.com>,
Paul Mackerras <paulus@samba.org>,
Ankita Garg <ankita@in.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries
Date: Wed, 01 Sep 2010 13:42:16 -0700 [thread overview]
Message-ID: <4C7EBAA8.7030601@us.ibm.com> (raw)
In-Reply-To: <1283371161.2356.53.camel@gandalf.stny.rr.com>
On 09/01/2010 12:59 PM, Steven Rostedt wrote:
> On Wed, 2010-09-01 at 11:47 -0700, Darren Hart wrote:
>
>> from tip/rt/2.6.33 causes the preempt_count() to change across the cede
>> call. This patch appears to prevents the proxy preempt_count assignment
>> from happening. This non-local-cpu assignment to 0 would cause an
>> underrun of preempt_count() if the local-cpu had disabled preemption
>> prior to the assignment and then later tried to enable it. This appears
>> to be the case with the stack of __trace_hcall* calls preceeding the
>> return from extended_cede_processor() in the latency format trace-cmd
>> report:
>>
>> <idle>-0 1d.... 201.252737: function: .cpu_die
>
> Note, the above 1d.... is a series of values. The first being the CPU,
> the next if interrupts are disabled, the next if the NEED_RESCHED flag
> is set, the next is softirqs enabled or disabled, next the
> preempt_count, and finally the lockdepth count.
>
> Here we only care about the preempt_count, which is zero when '.' and a
> number if it is something else. It is the second to last field in that
> list.
>
>
>> <idle>-0 1d.... 201.252738: function: .pseries_mach_cpu_die
>> <idle>-0 1d.... 201.252740: function: .idle_task_exit
>> <idle>-0 1d.... 201.252741: function: .switch_slb
>> <idle>-0 1d.... 201.252742: function: .xics_teardown_cpu
>> <idle>-0 1d.... 201.252743: function: .xics_set_cpu_priority
>> <idle>-0 1d.... 201.252744: function: .__trace_hcall_entry
>> <idle>-0 1d..1. 201.252745: function: .probe_hcall_entry
>
> ^
> preempt_count set to 1
>
>> <idle>-0 1d..1. 201.252746: function: .__trace_hcall_exit
>> <idle>-0 1d..2. 201.252747: function: .probe_hcall_exit
>> <idle>-0 1d.... 201.252748: function: .__trace_hcall_entry
>> <idle>-0 1d..1. 201.252748: function: .probe_hcall_entry
>> <idle>-0 1d..1. 201.252750: function: .__trace_hcall_exit
>> <idle>-0 1d..2. 201.252751: function: .probe_hcall_exit
>> <idle>-0 1d.... 201.252752: function: .__trace_hcall_entry
>> <idle>-0 1d..1. 201.252753: function: .probe_hcall_entry
> ^ ^
> CPU preempt_count
>
> Entering the function probe_hcall_entry() the preempt_count is 1 (see
> below). But probe_hcall_entry does:
>
> h = &get_cpu_var(hcall_stats)[opcode / 4];
>
> Without doing the put (which it does in probe_hcall_exit())
>
> So exiting the probe_hcall_entry() the prempt_count is 2.
> The trace_hcall_entry() will do a preempt_enable() making it leave as 1.
>
>
>> offon.sh-3684 6..... 201.466488: bprint: .smp_pSeries_kick_cpu : resetting pcnt to 0 for cpu 1
>
> This is CPU 6, changing the preempt count from 1 to 0.
>
>>
>> preempt_count() is reset from 1 to 0 by smp_startup_cpu() without the
>> QCSS_NOT_STOPPED check from the patch above.
>>
>> <idle>-0 1d.... 201.466503: function: .__trace_hcall_exit
>
> Note: __trace_hcall_exit() and __trace_hcall_entry() basically do:
>
> preempt_disable();
> call probe();
> preempt_enable();
>
>
>> <idle>-0 1d..1. 201.466505: function: .probe_hcall_exit
>
> The preempt_count of 1 entering the probe_hcall_exit() is because of the
> preempt_disable() shown above. It should have been entered as a 2.
>
> But then it does:
>
>
> put_cpu_var(hcall_stats);
>
> making preempt_count 0.
>
> But the preempt_enable() in the trace_hcall_exit() causes this to be -1.
>
>
>> <idle>-0 1d.Hff. 201.466507: bprint: .pseries_mach_cpu_die : after cede: ffffffff
>>
>> With the preempt_count() being one less than it should be, the final
>> preempt_enable() in the trace_hcall path drops preempt_count to
>> 0xffffffff, which of course is an illegal value and leads to a crash.
>
> I'm confused to how this works in mainline?
Turns out it didn't. 2.6.33.5 with CONFIG_PREEMPT=y sees this exact same
behavior. The following, part of the 2.6.33.6 stable release, prevents
this from happening:
aef40e87d866355ffd279ab21021de733242d0d5
powerpc/pseries: Only call start-cpu when a CPU is stopped
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -82,6 +82,12 @@ static inline int __devinit smp_startup_cpu(unsigned
int lcpu)
pcpu = get_hard_smp_processor_id(lcpu);
+ /* Check to see if the CPU out of FW already for kexec */
+ if (smp_query_cpu_stopped(pcpu) == QCSS_NOT_STOPPED){
+ cpu_set(lcpu, of_spin_map);
+ return 1;
+ }
+
/* Fixup atomic count: it exited inside IRQ handler. */
task_thread_info(paca[lcpu].__current)->preempt_count = 0;
The question is now, Is this the right fix? If so, perhaps we can update
the comment to be a bit more clear and not refer solely to kexec.
Michael Neuling, can you offer any thoughts here? We hit this EVERY
TIME, which makes me wonder if the offline/online path could do this
without calling smp_startup_cpu at all.
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
next prev parent reply other threads:[~2010-09-01 20:42 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-07-22 18:24 [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries Darren Hart
2010-07-22 18:36 ` Darren Hart
2010-07-22 18:38 ` Thomas Gleixner
2010-08-10 22:36 ` Darren Hart
2010-07-22 22:25 ` Benjamin Herrenschmidt
2010-07-22 23:57 ` Darren Hart
2010-07-23 4:44 ` Darren Hart
2010-07-23 5:08 ` Vaidyanathan Srinivasan
2010-07-23 5:11 ` Benjamin Herrenschmidt
2010-07-23 7:07 ` Vaidyanathan Srinivasan
2010-08-05 4:45 ` Darren Hart
2010-08-05 11:06 ` Vaidyanathan Srinivasan
2010-08-05 12:26 ` Thomas Gleixner
2010-07-23 5:09 ` Benjamin Herrenschmidt
2010-08-06 2:19 ` Darren Hart
2010-08-06 5:09 ` Vaidyanathan Srinivasan
2010-08-06 7:13 ` Darren Hart
2010-07-23 14:39 ` Will Schmidt
2010-08-04 13:44 ` Darren Hart
2010-08-19 15:58 ` Ankita Garg
2010-08-19 18:58 ` Will Schmidt
2010-08-23 22:20 ` Darren Hart
2010-08-31 7:12 ` Darren Hart
2010-09-01 5:54 ` Michael Ellerman
2010-09-01 15:10 ` Darren Hart
2010-09-01 18:47 ` Darren Hart
2010-09-01 19:59 ` Steven Rostedt
2010-09-01 20:42 ` Darren Hart [this message]
2010-09-02 1:02 ` Michael Neuling
2010-09-02 4:06 ` Steven Rostedt
2010-09-02 6:04 ` Darren Hart
2010-09-03 20:10 ` Will Schmidt
2010-09-02 23:04 ` Michael Neuling
2010-09-03 0:08 ` Darren Hart
2010-09-02 3:46 ` Michael Neuling
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