With Sub-NUMA Cluster mode enabled the scope of monitoring resources is per-NODE instead of per-L3 cache. Suffixes of directories with "L3" in their name refer to Sub-NUMA nodes instead of L3 cache ids. Users should be aware that SNC mode also affects the amount of L3 cache available for allocation within each SNC node. Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx> --- Changes since v5: Added addtional details about challenges tracking tasks when SNC mode is enabled. --- Documentation/arch/x86/resctrl.rst | 34 +++++++++++++++++++++++++++--- 1 file changed, 31 insertions(+), 3 deletions(-) diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst index cb05d90111b4..d6b6a4cfd967 100644 --- a/Documentation/arch/x86/resctrl.rst +++ b/Documentation/arch/x86/resctrl.rst @@ -345,9 +345,15 @@ When control is enabled all CTRL_MON groups will also contain: When monitoring is enabled all MON groups will also contain: "mon_data": - This contains a set of files organized by L3 domain and by - RDT event. E.g. on a system with two L3 domains there will - be subdirectories "mon_L3_00" and "mon_L3_01". Each of these + This contains a set of files organized by L3 domain or by NUMA + node (depending on whether Sub-NUMA Cluster (SNC) mode is disabled + or enabled respectively) and by RDT event. E.g. on a system with + SNC mode disabled with two L3 domains there will be subdirectories + "mon_L3_00" and "mon_L3_01". The numerical suffix refers to the + L3 cache id. With SNC enabled the directory names are the same, + but the numerical suffix refers to the node id. + Mappings from node ids to CPUs are available in the + /sys/devices/system/node/node*/cpulist files. Each of these directories have one file per event (e.g. "llc_occupancy", "mbm_total_bytes", and "mbm_local_bytes"). In a MON group these files provide a read out of the current value of the event for @@ -452,6 +458,28 @@ and 0xA are not. On a system with a 20-bit mask each bit represents 5% of the capacity of the cache. You could partition the cache into four equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000. +Notes on Sub-NUMA Cluster mode +============================== +When SNC mode is enabled the "llc_occupancy", "mbm_total_bytes", and +"mbm_local_bytes" will only give meaningful results for well behaved NUMA +applications. I.e. those that perform the majority of memory accesses +to memory on the local NUMA node to the CPU where the task is executing. +Note that Linux may load balance tasks between Sub-NUMA nodes much +more readily than between regular NUMA nodes since the CPUs on SNC +share the same L3 cache and the system may report the NUMA distance +between SNC nodes with a lower value than used for regular NUMA nodes. +Tasks that migrate between nodes will have their traffic recorded by the +counters in different SNC nodes so a user will need to read mon_data +files from each node on which the task executed to get the full +view of traffic for which the task was the source. + + +The cache allocation feature still provides the same number of +bits in a mask to control allocation into the L3 cache. But each +of those ways has its capacity reduced because the cache is divided +between the SNC nodes. The values reported in the resctrl +"size" files are adjusted accordingly. + Memory bandwidth Allocation and monitoring ========================================== -- 2.41.0
