Hi Tony, On Thu, Sep 28, 2023 at 9:14 PM Tony Luck <tony.luck@xxxxxxxxx> wrote: > 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 The explanation of mon_data seems overwhelmingly SNC-centric now. Maybe the SNC section should be responsible for explaining its impact on the mon_data directory. Mainly by reminding the reader that domain ids in the mon_data directory are node ids in SNC mode. > 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. Not being specific about why the results aren't meaningful, this sounds vague and alarming. > +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 > Reviewed-by: Peter Newman <peternewman@xxxxxxxxxx>
