Re: [PATCH v2 2/9] x86: numa: check the node id consistently for x86

From: Yunsheng Lin <linyunsheng@huawei.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: dalias@libc.org, linux-sh@vger.kernel.org,
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	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH v2 2/9] x86: numa: check the node id consistently for x86
Date: Tue, 03 Sep 2019 08:31:11 +0000	[thread overview]
Message-ID: <06eee8d0-ce56-03da-30a5-6b07e989a5e0@huawei.com> (raw)
In-Reply-To: <20190903071111.GU2369@hirez.programming.kicks-ass.net>

On 2019/9/3 15:11, Peter Zijlstra wrote:
> On Tue, Sep 03, 2019 at 02:19:04PM +0800, Yunsheng Lin wrote:
>> On 2019/9/2 20:56, Peter Zijlstra wrote:
>>> On Mon, Sep 02, 2019 at 08:25:24PM +0800, Yunsheng Lin wrote:
>>>> On 2019/9/2 15:25, Peter Zijlstra wrote:
>>>>> On Mon, Sep 02, 2019 at 01:46:51PM +0800, Yunsheng Lin wrote:
>>>>>> On 2019/9/1 0:12, Peter Zijlstra wrote:
>>>>>
>>>>>>> 1) because even it is not set, the device really does belong to a node.
>>>>>>> It is impossible a device will have magic uniform access to memory when
>>>>>>> CPUs cannot.
>>>>>>
>>>>>> So it means dev_to_node() will return either NUMA_NO_NODE or a
>>>>>> valid node id?
>>>>>
>>>>> NUMA_NO_NODE := -1, which is not a valid node number. It is also, like I
>>>>> said, not a valid device location on a NUMA system.
>>>>>
>>>>> Just because ACPI/BIOS is shit, doesn't mean the device doesn't have a
>>>>> node association. It just means we don't know and might have to guess.
>>>>
>>>> How do we guess the device's location when ACPI/BIOS does not set it?
>>>
>>> See device_add(), it looks to the device's parent and on NO_NODE, puts
>>> it there.
>>>
>>> Lacking any hints, just stick it to node0 and print a FW_BUG or
>>> something.
>>>
>>>> It seems dev_to_node() does not do anything about that and leave the
>>>> job to the caller or whatever function that get called with its return
>>>> value, such as cpumask_of_node().
>>>
>>> Well, dev_to_node() doesn't do anything; nor should it. It are the
>>> callers of set_dev_node() that should be taking care.
>>>
>>> Also note how device_add() sets the device node to the parent device's
>>> node on NUMA_NO_NODE. Arguably we should change it to complain when it
>>> finds NUMA_NO_NODE and !parent.
>>
>> Is it possible that the node id set by device_add() become invalid
>> if the node is offlined, then dev_to_node() may return a invalid
>> node id.
> 
> In that case I would expect the device to go away too. Once the memory
> controller goes away, the PCI bus connected to it cannot continue to
> function.

Ok. To summarize the discussion in order to for me to understand it
correctly:

1) Make sure device_add() set to default node0 to a device if
   ACPI/BIOS does not set the node id and it has not no parent device.

2) Use '(unsigned)node_id >= nr_node_ids' to fix the
   CONFIG_DEBUG_PER_CPU_MAPS version of cpumask_of_node() for x86
   and arm64, x86 just has a fix from you now, a patch for arm64 is
   also needed.

3) Maybe fix some other the sign bug for node id checking through the
   kernel using the '(unsigned)node_id >= nr_node_ids'.

Please see if I understand it correctly or miss something.
Maybe I can begin by sending a patch about item one to see if everyone
is ok with the idea?

> 
>> From the comment in select_fallback_rq(), it seems that a node can
>> be offlined, not sure if node offline process has taken cared of that?
>>
>> 	/*
>>          * If the node that the CPU is on has been offlined, cpu_to_node()
>>          * will return -1. There is no CPU on the node, and we should
>>          * select the CPU on the other node.
>>          */
> 
> Ugh, so I disagree with that notion. cpu_to_node() mapping should be
> fixed, you simply cannot change it after boot, too much stuff relies on
> it.
> 
> Setting cpu_to_node to -1 on node offline is just wrong. But alas, it
> seems this is already so.