On 2015/8/20 8:00, David Rientjes wrote: > On Wed, 19 Aug 2015, Jiang Liu wrote: > >> On 2015/8/18 8:31, David Rientjes wrote: >>> On Mon, 17 Aug 2015, Jiang Liu wrote: >>> >>>> Function profile_cpu_callback() allocates memory without specifying >>>> __GFP_THISNODE flag, so replace cpu_to_mem() with cpu_to_node() >>>> because cpu_to_mem() may cause suboptimal memory allocation if >>>> there's no free memory on the node returned by cpu_to_mem(). >>>> >>> >>> Why is cpu_to_node() better with regard to free memory and NUMA locality? >> Hi David, >> Thanks for review. This is a special case pointed out by Tejun. >> 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. > > Ok, that makes sense, but I would prefer that this > alloc_pages_exact_node() change to alloc_pages_node() since, as you > mention in your commit message, __GFP_THISNODE is not set. Hi David, Sorry for slow response due to personal reasons! Function alloc_pages_exact_node() has been renamed as __alloc_pages_node() by commit 96db800f5d73, and __alloc_pages_node() is a slightly optimized version of alloc_pages_node() which doesn't fallback to current node for nid == NUMA_NO_NODE case. So it would be better to keep using __alloc_pages_node() because cpu_to_node() always returns valid node id. Thanks! Gerry > > In the longterm, if we setup both zonelists correctly (no __GFP_THISNODE > and with __GFP_THISNODE), then I'm not sure there's any reason to ever use > cpu_to_mem() for alloc_pages(). > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- 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
