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. > 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. > 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?
