Hi Tony, On 9/12/2023 9:01 AM, Tony Luck wrote: > On Mon, Sep 11, 2023 at 01:23:35PM -0700, Reinette Chatre wrote: >> Hi Tony, >> >> On 8/29/2023 4:44 PM, Tony Luck wrote: >>> The Sub-NUMA cluster feature on some Intel processors partitions >>> the CPUs that share an L3 cache into two or more sets. This plays >>> havoc with the Resource Director Technology (RDT) monitoring features. >>> Prior to this patch Intel has advised that SNC and RDT are incompatible. >>> >>> Some of these CPU support an MSR that can partition the RMID >>> counters in the same way. This allows for monitoring features >>> to be used (with the caveat that memory accesses between different >>> SNC NUMA nodes may still not be counted accuratlely. >> >> Same typo as in V4. > > Sorry. Will fix and re-post. > >>> >>> Note that this patch series improves resctrl reporting considerably >>> on systems with SNC enabled, but there will still be some anomalies >>> for processes accessing memory from other sub-NUMA nodes. >> >> I have the same question as with V4 that was not answered in that email >> thread nor in this new version. >> https://lore.kernel.org/lkml/e350514e-76ed-14ea-3e74-c0852658182f@xxxxxxxxx/ > > Non-SNC systems already have an issue when reporting memory bandwidth > for a task that Linux may migrate the task to a CPU on a different node > which means that logging for that task will also move to different files > in the mon_data/mon_L3_*/ for the new node. It is not obvious to me that this is an issue. From what I understand the data remains accurate. How does this map to the earlier "may still not be counted accurately"? > > With SNC enabled, migration between NUMA nodes on the same socket may happen > much more frequently because: > 1) The CPUs on the other NUMA nodes in the socket are in the same Linux > L3 cache domain. So Linux regard the migration as "cheap". > 2) The ACPI SLIT table on SNC enabled systems may also report the > latency for remote access to another NUMA node on the same socket > as significantly lower than the latency for cross-socket access. On > my test system the SLIT distance for same socket nodes is 0xC, > compared to 0x15 for cross-socket distance. This will also lead > to Linux being more likely to migrate a task to a CPU on another > SNC NUMA node in the same socket. > > To avoid migration issues, users may use sched_setaffinity(2) to bind > tasks to the subset of CPUs that share an SNC NUMA node. > > I can write this up in a new cover letter. > >> I stop my review of this series here. > > Reinette > > Should I repost the whole series as v6 with the new cover letter. The > only change to the patches so far is to the selftest reported by > Shaopeng Tan[1]. > Is this an assurance that the cover letter in no way reflects how feedback was addressed in the rest of this series? Reinette
