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 13:50:02 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>,
        "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>
CC:     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

Hi Tony,

On 6/29/2023 9:05 AM, Luck, Tony wrote:
>> I ran selftest/resctrl in my environment,
>> CMT test failed when I enabled Sub-NUMA Cluster.
>>
>> I don't know why it failed yet,
>> I paste the test results below.
>>
>> Processer in my environment:
>> Intel(R) Xeon(R) Gold 6254 CPU @ 3.10GHz
>>
>> $ sudo make -C tools/testing/selftests/resctrl run_tests
>> # # Starting CMT test ...
>> # # Mounting resctrl to "/sys/fs/resctrl"
>> # # Mounting resctrl to "/sys/fs/resctrl"
>> # # Cache size :25952256
>> # # Benchmark PID: 8638
>> # # Writing benchmark parameters to resctrl FS
>> # # Checking for pass/fail
>> # # Fail: Check cache miss rate within 15%
>> # # Percent diff=21
>> # # Number of bits: 5
>> # # Average LLC val: 9216000
>> # # Cache span (bytes): 11796480
>> # not ok 3 CMT: test
> 
> 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?

Reinette