Subject: Re: [PATCH] power, sched: stop updating inside arch_update_cpu_topology() when nothing to be update

From: Michael wang <[email-masked]>
Date: Tue, 08 Apr 2014 10:39:31 +0800

Hi, Srivatsa

It's nice to have you confirmed the fix, and thanks for the well-writing
comments, will apply them and send out the new patch later :)

Regards,
Michael Wang

On 04/07/2014 06:15 PM, Srivatsa S. Bhat wrote:
> Hi Michael,
> 
> On 04/02/2014 08:59 AM, Michael wang wrote:
>> During the testing, we encounter below WARN followed by Oops:
>>
>> 	WARNING: at kernel/sched/core.c:6218
>> 	...
>> 	NIP [c000000000101660] .build_sched_domains+0x11d0/0x1200
>> 	LR [c000000000101358] .build_sched_domains+0xec8/0x1200
>> 	PACATMSCRATCH [800000000000f032]
>> 	Call Trace:
>> 	[c00000001b103850] [c000000000101358] .build_sched_domains+0xec8/0x1200
>> 	[c00000001b1039a0] [c00000000010aad4] .partition_sched_domains+0x484/0x510
>> 	[c00000001b103aa0] [c00000000016d0a8] .rebuild_sched_domains+0x68/0xa0
>> 	[c00000001b103b30] [c00000000005cbf0] .topology_work_fn+0x10/0x30
>> 	...
>> 	Oops: Kernel access of bad area, sig: 11 [#1]
>> 	...
>> 	NIP [c00000000045c000] .__bitmap_weight+0x60/0xf0
>> 	LR [c00000000010132c] .build_sched_domains+0xe9c/0x1200
>> 	PACATMSCRATCH [8000000000029032]
>> 	Call Trace:
>> 	[c00000001b1037a0] [c000000000288ff4] .kmem_cache_alloc_node_trace+0x184/0x3a0
>> 	[c00000001b103850] [c00000000010132c] .build_sched_domains+0xe9c/0x1200
>> 	[c00000001b1039a0] [c00000000010aad4] .partition_sched_domains+0x484/0x510
>> 	[c00000001b103aa0] [c00000000016d0a8] .rebuild_sched_domains+0x68/0xa0
>> 	[c00000001b103b30] [c00000000005cbf0] .topology_work_fn+0x10/0x30
>> 	...
>>
>> This was caused by that 'sd->groups == NULL' after building groups, which
>> was caused by the empty 'sd->span'.
>>
>> The cpu's domain contain nothing because the cpu was assigned to wrong
>> node inside arch_update_cpu_topology() by calling update_lookup_table()
>> with the uninitialized param, in the case when there is nothing to be
>> update.
>>
> 
> Can you reword the above paragraph to something like this:
> 
> The cpu's domain contained nothing because the cpu was assigned to a wrong
> node, due to the following unfortunate sequence of events:
> 
> 1. The hypervisor sent a topology update to the guest OS, to notify changes
>    to the cpu-node mapping. However, the update was actually redundant - i.e.,
>    the "new" mapping was exactly the same as the old one.
> 
> 2. Due to this, the 'updated_cpus' mask turned out to be empty after exiting
>    the 'for-loop' in arch_update_cpu_topology().
> 
> 3. So we ended up calling stop-machine() with an empty cpumask list, which made
>    stop-machine internally elect cpumask_first(cpu_online_mask), i.e., CPU0 as
>    the cpu to run the payload (the update_cpu_topology() function).
> 
> 4. This causes update_cpu_topology() to be run by CPU0. And since 'updates'
>    is kzalloc()'ed inside arch_update_cpu_topology(), update_cpu_topology()
>    finds update->cpu as well as update->new_nid to be 0. In other words, we
>    end up assigning CPU0 (and eventually its siblings) to node 0, incorrectly.
> 
> This causes the sched-domain rebuild code to break and crash the system.
> 
> 
>> Thus we should stop the updating in such cases, this patch will achieve
>> this and fix the issue.
>>
> 
> We can reword this part as:
> 
> Fix this by skipping the topology update in cases where we find that
> the topology has not actually changed in reality (ie., spurious updates).
> 
>> CC: Benjamin Herrenschmidt <[email-masked]>
>> CC: Paul Mackerras <[email-masked]>
>> CC: Nathan Fontenot <[email-masked]>
>> CC: Stephen Rothwell <[email-masked]>
>> CC: Andrew Morton <[email-masked]>
>> CC: Robert Jennings <[email-masked]>
>> CC: Jesse Larrew <[email-masked]>
>> CC: "Srivatsa S. Bhat" <[email-masked]>
>> CC: Alistair Popple <[email-masked]>
>> Signed-off-by: Michael Wang <[email-masked]>
>> ---
>>  arch/powerpc/mm/numa.c |    9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>> index 30a42e2..6757690 100644
>> --- a/arch/powerpc/mm/numa.c
>> +++ b/arch/powerpc/mm/numa.c
>> @@ -1591,6 +1591,14 @@ int arch_update_cpu_topology(void)
>>  		cpu = cpu_last_thread_sibling(cpu);
>>  	}
>>  
>> +	/*
>> +	 * The 'cpu_associativity_changes_mask' could be cleared if
>> +	 * all the cpus it indicates won't change their node, in
>> +	 * which case the 'updated_cpus' will be empty.
>> +	 */
> 
> How about rewording the comment like this:
> 
> In cases where we have nothing to update (because the updates list
> is too short or because the new topology is same as the old one),
> skip invoking update_cpu_topology() via stop-machine(). This is
> necessary (and not just a fast-path optimization) because stop-machine
> can end up electing a random CPU to run update_cpu_topology(), and
> thus trick us into setting up incorrect cpu-node mappings (since
> 'updates' is kzalloc()'ed).
> 
> Regards,
> Srivatsa S. Bhat
> 
>> +	if (!cpumask_weight(&updated_cpus))
>> +		goto out;
>> +
>>  	stop_machine(update_cpu_topology, &updates[0], &updated_cpus);
>>  
>>  	/*
>> @@ -1612,6 +1620,7 @@ int arch_update_cpu_topology(void)
>>  		changed = 1;
>>  	}
>>  
>> +out:
>>  	kfree(updates);
>>  	return changed;
>>  }
>>
> 
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