powerpc/pseries: Add documentation for vcpudispatch_stats

Add a document describing the fields provided by
/proc/powerpc/vcpudispatch_stats.
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/powerpc/vcpudispatch_stats.txt

+VCPU Dispatch Statistics:
+=========================
+
+For Shared Processor LPARs, the POWER Hypervisor maintains a relatively
+static mapping of the LPAR processors (vcpus) to physical processor
+chips (representing the "home" node) and tries to always dispatch vcpus
+on their associated physical processor chip. However, under certain
+scenarios, vcpus may be dispatched on a different processor chip (away
+from its home node).
+
+/proc/powerpc/vcpudispatch_stats can be used to obtain statistics
+related to the vcpu dispatch behavior. Writing '1' to this file enables
+collecting the statistics, while writing '0' disables the statistics.
+By default, the DTLB log for each vcpu is processed 50 times a second so
+as not to miss any entries. This processing frequency can be changed
+through /proc/powerpc/vcpudispatch_stats_freq.
+
+The statistics themselves are available by reading the procfs file
+/proc/powerpc/vcpudispatch_stats. Each line in the output corresponds to
+a vcpu as represented by the first field, followed by 8 numbers.
+
+The first number corresponds to:
+1. total vcpu dispatches since the beginning of statistics collection
+
+The next 4 numbers represent vcpu dispatch dispersions:
+2. number of times this vcpu was dispatched on the same processor as last
+   time
+3. number of times this vcpu was dispatched on a different processor core
+   as last time, but within the same chip
+4. number of times this vcpu was dispatched on a different chip
+5. number of times this vcpu was dispatches on a different socket/drawer
+(next numa boundary)
+
+The final 3 numbers represent statistics in relation to the home node of
+the vcpu:
+6. number of times this vcpu was dispatched in its home node (chip)
+7. number of times this vcpu was dispatched in a different node
+8. number of times this vcpu was dispatched in a node further away (numa
+distance)
+
+An example output:
+    $ sudo cat /proc/powerpc/vcpudispatch_stats
+    cpu0 6839 4126 2683 30 0 6821 18 0
+    cpu1 2515 1274 1229 12 0 2509 6 0
+    cpu2 2317 1198 1109 10 0 2312 5 0
+    cpu3 2259 1165 1088 6 0 2256 3 0
+    cpu4 2205 1143 1056 6 0 2202 3 0
+    cpu5 2165 1121 1038 6 0 2162 3 0
+    cpu6 2183 1127 1050 6 0 2180 3 0
+    cpu7 2193 1133 1052 8 0 2187 6 0
+    cpu8 2165 1115 1032 18 0 2156 9 0
+    cpu9 2301 1252 1033 16 0 2293 8 0
+    cpu10 2197 1138 1041 18 0 2187 10 0
+    cpu11 2273 1185 1062 26 0 2260 13 0
+    cpu12 2186 1125 1043 18 0 2177 9 0
+    cpu13 2161 1115 1030 16 0 2153 8 0
+    cpu14 2206 1153 1033 20 0 2196 10 0
+    cpu15 2163 1115 1032 16 0 2155 8 0
+
+In the output above, for vcpu0, there have been 6839 dispatches since
+statistics were enabled. 4126 of those dispatches were on the same
+physical cpu as the last time. 2683 were on a different core, but within
+the same chip, while 30 dispatches were on a different chip compared to
+its last dispatch.
+
+Also, out of the total of 6839 dispatches, we see that there have been
+6821 dispatches on the vcpu's home node, while 18 dispatches were
+outside its home node, on a neighbouring chip.