linux-kernel - Re: [PATCH v2 0/7] x86/resctrl: Add support for Sub-NUMA cluster (SNC) systems

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Date:   Tue, 11 Jul 2023 14:23:14 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "x86@...nel.org" <x86@...nel.org>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH v2 0/7] x86/resctrl: Add support for Sub-NUMA cluster
 (SNC) systems

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