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

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Date: Fri, 31 May 2024 09:17:46 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, "Wieczor-Retman, Maciej"
	<maciej.wieczor-retman@...el.com>, "Yu, Fenghua" <fenghua.yu@...el.com>,
	"shuah@...nel.org" <shuah@...nel.org>
CC: "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ilpo.jarvinen@...ux.intel.com" <ilpo.jarvinen@...ux.intel.com>
Subject: Re: [PATCH v2 1/2] selftests/resctrl: Adjust effective L3 cache size
 with SNC enabled


Hi Tony and Maciej,

On 5/30/24 5:34 PM, Luck, Tony wrote:
>>>>> When SNC mode is enabled the effective amount of L3 cache available
>>>>> for allocation is divided by the number of nodes per L3.
>>>>
>>>> This was a mistake in original implementation and no longer done.
>>>
>>> My original kernel code adjusted value reported in the "size" file in resctrl.
>>> That's no longer done because the effective size depends on how applications
>>> are allocating and using memory. Since the kernel can't know that, it
>>> seemed best to just report the total size of the cache.
>>>
>>> But I think the resctrl tests still need to take this into account when running
>>> llc_occupancy tests.
>>>
>>> E.g. on a 2-way SNC system with a 100MB L3 cache a test that allocates
>>> memory from its local SNC node (default behavior without using libnuma)
>>> will only see 50 MB llc_occupancy with a fully populated L3 mask in the
>>> schemata file.
>>
>> This seems to contradict the "Cache and memory bandwidth allocation features
>> continue to operate at the scope of the L3 cache." statement from [1]?
> 
> I'll clean that up. MBA isn't affected. But cache allocation is affected in that
> the amount of cache represented by each bit in the masks in the schemata
> file is reduced by a factor equal to SNC nodes per L3 cache.

Thanks Tony. I trust that this is what Maciej intended since the change
is specifically named "Adjuct _effective_ L3 cache size". I'd like to
recommend that your comments be added before the change to
get_cache_size() ...

	/*
	 * The amount of cache represented by each bit in the masks
	 * in the schemata file is reduced by a factor equal to SNC
	 * nodes per L3 cache.
	 * E.g. on a SNC-2 system with a 100MB L3 cache a test that
	 * allocates memory from its local SNC node (default behavior
	 * without using libnuma) will only see 50 MB llc_occupancy
	 * with a fully populated L3 mask in the schemata file.
	 */

Reinette