Hi Tony, On 9/12/2023 10:45 AM, Tony Luck wrote: > On Tue, Sep 12, 2023 at 10:13:31AM -0700, Reinette Chatre wrote: >> 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"? > > Yes, the data is accurate. But a naive application reading the wrong > files from mon_data will not see the accurate data. Without SNC users > may only see this issue rarely as Linux tries hard to not migrate > processes to other NUMA nodes or L3 cache domains. But with SNC enabled > this is no longer the case. the ACPI SLIT distance of 0xC is below the > threshold that Linux checks for "is the target CPU for a migration far away" > so migration to other SNC nodes may be quite common. I would like to recommend that you take care not to present this work using uncertain terms like "may not be counted accurately" or "there will still be some anomalies". If I understand correctly there is no uncertainty. When that "naive application" reads the wrong files then I think it can be considered a usage error and should not be documented as an issue with the counters. I understand that the user space requirements are not obvious, and there should be guidance. > I can move this out of the cover letter and provide guidance/warnings > in the patch to Documentation/arch/x86/resctrl.rst Yes, I do think this will be very helpful. > >> >>> >>> 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? > > My track record here is far from perfect. I believe I addressed all > the issues you raised. But it's very possible that I may have missed > some, or misintepreted the concerns you raised. This is a familiar response from you that just puts the burden back on me to go dig out previous discussions. This will be the last time. Reinette
