linux-kernel - Re: [PATCH v3 22/29] selftests/resctrl: Rewrite Cache Allocation Technology (CAT) test

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Date:   Wed, 13 Dec 2023 13:56:58 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        "Shaopeng Tan" <tan.shaopeng@...fujitsu.com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 22/29] selftests/resctrl: Rewrite Cache Allocation
 Technology (CAT) test

Hi Ilpo,

On 12/11/2023 4:18 AM, Ilpo Järvinen wrote:
> CAT test spawns two processes into two different control groups with
> exclusive schemata. Both the processes alloc a buffer from memory
> matching their allocated LLC block size and flush the entire buffer out
> of caches. Since the processes are reading through the buffer only once
> during the measurement and initially all the buffer was flushed, the
> test isn't testing CAT.
> 
> Rewrite the CAT test to allocate a buffer sized to half of LLC. Then
> perform a sequence of tests with different LLC alloc sizes starting
> from half of the CBM bits down to 1-bit CBM. Flush the buffer before
> each test and read the buffer twice. Observe the LLC misses on the
> second read through the buffer. As the allocated LLC block gets smaller
> and smaller, the LLC misses will become larger and larger giving a
> strong signal on CAT working properly.
> 
> The new CAT test is using only a single process because it relies on
> measured effect against another run of itself rather than another
> process adding noise. The rest of the system is set to use the CBM bits
> not used by the CAT test to keep the test isolated.
> 
> Replace count_bits() with count_contiguous_bits() to get the first bit
> position in order to be able to calculate masks based on it.
> 
> This change has been tested with a number of systems from different
> generations.
> 
> Suggested-by: Reinette Chatre <reinette.chatre@...el.com>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---

Thank you!

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette