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

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Message-ID: <1f48081c-c9d6-8f3e-9559-8b0bec98f125@huawei.com>
Date:   Tue, 3 Sep 2019 14:19:04 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <dalias@...c.org>, <linux-sh@...r.kernel.org>,
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Subject: Re: [PATCH v2 2/9] x86: numa: check the node id consistently for x86

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.

>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.
         */


With the above assumption that a device is always on a valid node,
the node id returned from dev_to_node() can be safely passed to
cpumask_of_node() without any checking?

> 
> ---
>  drivers/base/core.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index f0dd8e38fee3..2caf204966a0 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -2120,8 +2120,16 @@ int device_add(struct device *dev)
>  		dev->kobj.parent = kobj;
>  
>  	/* use parent numa_node */
> -	if (parent && (dev_to_node(dev) == NUMA_NO_NODE))
> -		set_dev_node(dev, dev_to_node(parent));
> +	if (dev_to_node(dev) == NUMA_NO_NODE) {
> +		if (parent)
> +			set_dev_node(dev, dev_to_node(parent));
> +#ifdef CONFIG_NUMA
> +		else {
> +			pr_err("device: '%s': has no assigned NUMA node\n", dev_name(dev));
> +			set_dev_node(dev, 0);
> +		}
> +#endif
> +	}
>  
>  	/* first, register with generic layer. */
>  	/* we require the name to be set before, and pass NULL */
> 
> .
>