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

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Date: Thu, 30 May 2024 16:51:33 -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,

On 5/30/24 4:46 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]?

Reinette

[1] https://lore.kernel.org/lkml/20240528222006.58283-1-tony.luck@intel.com/