linux-kernel - Re: 答复: [PATCH] fs/resctrl: fix domid loss precision issue

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Date: Mon, 11 Mar 2024 12:49:39 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Rex Nie <rex.nie@...uarmicro.com>
CC: "james.morse@....com" <james.morse@....com>, "fenghua.yu@...el.com"
	<fenghua.yu@...el.com>, "reinette.chatre@...el.com"
	<reinette.chatre@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Liming Wu <liming.wu@...uar.com>
Subject: Re: 答复: [PATCH] fs/resctrl: fix
 domid loss precision issue

Thanks for the reply,

On 2024-03-11 at 09:37:37 +0000, Rex Nie wrote:
>Hello,
>	Please kindly check my inline reply. Thanks.
>Best regards
>Rex Nie
>
>> >This will cause below issue if cache_id > 0x3fff likes:
>> 
>> Is there some reason for cache_id ever being this high?
>> 
>> I thought the max for cache_id was the amount of L3 caches on a system. And I
>> only observed it going up to 3 on some server platforms. So not nearly in the
>> range of 0x3fff or 16k.
>> 
>It is exactly as you said on X86 platforms, but cache_Id on Arm platform is different.
>According to ACPI for mpam, cache id is used as locator for cache MSC. Reference to RD_PPTT_CACHE_ID definition from edk2-platforms:
>#define RD_PPTT_CACHE_ID(PackageId, ClusterId, CoreId, CacheType)         \
>	(                                                              \
>	  (((PackageId) & 0xF) << 20) | (((ClusterId) & 0xFF) << 12) |              \
>	  (((CoreId) & 0xFF) << 4) | ((CacheType) & 0xF)                        \
>	)
>So it may be > 0x3fff on Arm platform.
>
>Reference RD_PPTT_CACHE_ID from edk2-platforms: https://github.com/tianocore/edk2-platforms/blob/master/Platform/ARM/SgiPkg/Include/SgiAcpiHeader.h#L202

and thanks for clearing it up for me! I browsed some MPAM patches but didn't
notice cache_id was used differently on ARM.

>
>> >/sys/fs/resctrl/mon_groups/p1/mon_data/mon_L3_1048564 # cat
>> >llc_occupancy
>> 
>> How did you get this file to appear? Could you maybe show how your
>> mon_data directory looks like?
>> 
>I found this issue on Arm FVP N1 platform and my N2 platform.
>
>Below is the steps on Arm FVP N1:
>mount -t resctrl resctrl / /sys/fs/resctrl
>cd /sys/fs/resctrl/mon_data
>
>/sys/fs/resctrl/mon_data # ls -l
>total 0
>dr-xr-xr-x    2 0        0                0 Mar 11 09:24 mon_L3_1048564
>
>cd /sys/fs/resctrl/mon_data # cd mon_L3_1048564
>/sys/fs/resctrl/mon_data/mon_L3_1048564 # cat llc_occupancy
>cat: read error: No such file or directory
>
>Arm FVP MPAM: https://neoverse-reference-design.docs.arm.com/en/latest/mpam/mpam-resctrl.html#memory-system-resource-partitioning-and-monitoring-mpam
>

-- 
Kind regards
Maciej Wieczór-Retman