On 11/26/18 7:38 AM, Anshuman Khandual wrote: > On 11/24/2018 12:51 AM, Dave Hansen wrote: >> On 11/22/18 10:42 PM, Anshuman Khandual wrote: >>> Are we willing to go in the direction for inclusion of a new system >>> call, subset of it appears on sysfs etc ? My primary concern is not >>> how the attribute information appears on the sysfs but lack of it's >>> completeness. >> >> A new system call makes total sense to me. I have the same concern >> about the completeness of what's exposed in sysfs, I just don't see a >> _route_ to completeness with sysfs itself. Thus, the minimalist >> approach as a first step. > > Okay if we agree on the need for a new specific system call extracting > the superset attribute information MAX_NUMNODES * MAX_NUMNODES * U64 > (u64 packs 8 bit values for 8 attributes or something like that) as we > had discussed before, it makes sense to export a subset of it which can > be faster but useful for the user space without going through a system > call. The information that needs to be exported is a bit more than that. It's not just a binary attribute. The information we have from the new ACPI table, for instance, is the read and write bandwidth and latency between two nodes. They are, IIRC, two-byte values in the ACPI table[1], each. That's 8 bytes worth of data right there, which wouldn't fit *anything* else. The list of things we want to export will certainly grow. That means we need a syscall something like this: int get_mem_attribute(unsigned long attribute_nr, unsigned long __user * initiator_nmask, unsigned long __user * target_nmask, unsigned long maxnode, unsigned long *attributes_out); #define MEM_ATTR_READ_BANDWIDTH 1 #define MEM_ATTR_WRITE_BANDWIDTH 2 #define MEM_ATTR_READ_LATENCY 3 #define MEM_ATTR_WRITE_LATENCTY 4 #define MEM_ATTR_ENCRYPTION 5 If you want to know the read latency between nodes 4 and 8, you do: ret = get_mem_attr(MEM_ATTR_READ_LATENCY, (1<<4), (1<<8), max, &array); And the answer shows up at array[0] in this example. If you had more than one bit set in the two nmasks, you would have a longer array. The length of the array is the number of bits set in initiator_nmask * the number of bits set in target_nmask * sizeof(ulong). This has the advantage of supporting ULONG_MAX attributes, and scales from asking for one attribute at a time all the way up to dumping the entire system worth of data for a single attribute. The only downside is that it's one syscall per attribute instead of packing them all together. But, if we have a small enough number to pack them in one ulong, then I think we can make 64 syscalls without too much trouble. > Do you agree on a (system call + sysfs) approach in principle ? > Also sysfs exported information has to be derived from whats available > through the system call not the other way round. Hence the starting > point has to be the system call definition. Both the sysfs information *and* what will be exported in any future interfaces are derived from platform-specific information. They are not derived from one _interface_ or the other. They obviously need to be consistent, though. 1. See "Table 5-142 System Locality Latency and Bandwidth Information Structure" here: http://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf
