linux-kernel - Re: [PATCH v2 1/2] selftests/resctrl: Adjust effective L3 cache size with SNC enabled

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Date: Thu, 27 Jun 2024 09:30:24 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
CC: <fenghua.yu@...el.com>, <shuah@...nel.org>,
	<linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<ilpo.jarvinen@...ux.intel.com>, <tony.luck@...el.com>
Subject: Re: [PATCH v2 1/2] selftests/resctrl: Adjust effective L3 cache size
 with SNC enabled

Hi Maciej

On 6/27/24 2:50 AM, Maciej Wieczor-Retman wrote:
> 
> Yeah, I've been thinking about what is the best way to display these for a
> while. Maybe you're right that messages at the top will be lost. What about this
> set of messages:
> 
> 1. First run of run_single_test()
> 	1.1. For all tests:
> 		- detected snc mode (if > 1)
> 			- check if cpu/offline file is empty, set the global
> 			  variable and print a message saying snc mode might be
> 			  wrong

When SNC detection is considered unreliable, everything else becomes unreliable also
since kernel support for SNC is only visible (in future kernels) when SNC is enabled.
I thus think that if it is found that SNC detection may be unreliable then the number
of SNC nodes should be hardcoded to 1 and a default message about possible interference
by SNC should be printed at all test failures.

> 2. At the end of tests
> 	2.1. For CMT, CAT, MBM, MBA:
> 		- test failed
> 		- snc detection reports it's enabled
> 		- kernel version doesn't support snc

Sounds like the "all goes well" scenario when SNC support is reliably detected.

> 
> 	2.2. Additional message for CMT, CAT (since the cache size is divided):
> 		- test failed or succeeded
> 		- snc detection reports the offline file is not empty
> 		- kernel version supports snc

I am not able to follow what happens in these scenarios.

> 
> The 1. message will be printed at the top since it's more informational (what is
> the SNC mode?) and then 2. messages will deal with possible issues / failures
> and will be nicely visible at the end. What do you think about this?

It is not obvious to me what the messages may be but the times/locations when
messages are printed sounds good to me.

Thank you

Reinette