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

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Date:   Thu, 29 Jun 2023 16:05:45 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...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

> 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).

-Tony