Messages in this thread | | | Date | Tue, 11 Jul 2023 14:23:14 -0700 | From | Tony Luck <> | Subject | Re: [PATCH v2 0/7] x86/resctrl: Add support for Sub-NUMA cluster (SNC) systems |
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On Tue, Jul 11, 2023 at 01:50:02PM -0700, Reinette Chatre wrote: > Hi Tony, > > This is expected. When SNC is enabled, CAT still supports the same number of > > bits in the allocation cache mask. But each bit represents half as much cache. > > > > Think of the cache as a 2-D matrix with the cache-ways (bits in the CAT mask) > > as the columns, and the rows are the hashed index of the physical address. > > When SNC is turned on the hash function for physical addresses from one > > of the SNC number nodes will only pick half of those rows (and the other > > SNC node gets the other half of the rows). > > If a test is expected to fail in a particular scenario then I think > the test failure should be communicated as a "pass". If not this will > reduce confidence in accuracy of tests. Even so, from the description > it sounds as though this test can be made more accurate to indeed pass > in the scenario when SNC is enabled?
Hi Reinette,
Yes. This could be done. The resctrl tests would need to determine if SNC mode is enabled. But I think that is possible by comparing output of sysfs files. E.g. with SNC disabled the lists of cpus for a node and a CPU on that node will match like this:
$ cat /sys/devices/system/node/node0/cpulist 0-35,72-107 $ cat /sys/devices/system/cpu/cpu0/cache/index3/shared_cpu_list 0-35,72-107
but with SNC enabled, the CPUs sharing a cache will be divided across two or four nodes.
It looks like the existing tests may print a warning. I see this code in:
tools/testing/selftests/resctrl/resctrl_tests.c
123 res = cmt_resctrl_val(cpu_no, 5, benchmark_cmd); 124 ksft_test_result(!res, "CMT: test\n"); 125 if ((get_vendor() == ARCH_INTEL) && res) 126 ksft_print_msg("Intel CMT may be inaccurate when Sub-NUMA Clustering is enabled. Check BIOS configuration.\n");
but at first glance that warning doesn't appear to try and check if SNC was the actual problem.
-Tony
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