On 2015/8/18 18:02, Tang Chen wrote: > > On 08/17/2015 11:18 AM, Jiang Liu wrote: >> This is the third version to enable memoryless node support on x86 >> platforms. The previous version (https://lkml.org/lkml/2014/7/11/75) >> blindly replaces numa_node_id()/cpu_to_node() with numa_mem_id()/ >> cpu_to_mem(). That's not the right solution as pointed out by Tejun >> and Peter due to: >> 1) We shouldn't shift the burden to normal slab users. >> 2) Details of memoryless node should be hidden in arch and mm code >> as much as possible. >> >> After digging into more code and documentation, we found the rules to >> deal with memoryless node should be: >> 1) Arch code should online corresponding NUMA node before onlining any >> CPU or memory, otherwise it may cause invalid memory access when >> accessing NODE_DATA(nid). >> 2) For normal memory allocations without __GFP_THISNODE setting in the >> gfp_flags, we should prefer numa_node_id()/cpu_to_node() instead of >> numa_mem_id()/cpu_to_mem() because the latter loses hardware topology >> information as pointed out by Tejun: >> A - B - X - C - D >> Where X is the memless node. numa_mem_id() on X would return >> either B or C, right? If B or C can't satisfy the allocation, >> the allocator would fallback to A from B and D for C, both of >> which aren't optimal. It should first fall back to C or B >> respectively, which the allocator can't do anymoe because the >> information is lost when the caller side performs numa_mem_id(). > > Hi Liu, > > BTW, how is this A - B - X - C - D problem solved ? > I don't quite follow this. > > I cannot tell the difference between numa_node_id()/cpu_to_node() and > numa_mem_id()/cpu_to_mem() on this point. Even with hardware topology > info, how could it avoid this problem ? > > Isn't it still possible falling back to A from B and D for C ? Hi Chen, For the imagined topology, A<->B<->X<->C<->D, where A, B, C, D has memory and X is memoryless. Possible fallback lists are: B: [ B, A, C, D] X: [ B, C, A, D] C: [ C, D, B, A] cpu_to_mem(X) will either return B or C. Let's assume it returns B. Then we will use "B: [ B, A, C, D]" to allocate memory for X, which is not the optimal fallback list for X. And cpu_to_node(X) returns X, and "X: [ B, C, A, D]" is the optimal fallback list for X. Thanks! Gerry -- To unsubscribe from this list: send the line "unsubscribe linux-hotplug" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
