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

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Message-ID: <dcef573e-dada-efdc-32d0-8e6070af9d99@linux.intel.com>
Date: Mon, 11 Mar 2024 14:17:48 +0200 (EET)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, 
    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

On Mon, 11 Mar 2024, Maciej Wieczor-Retman wrote:

> 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.

Hi Rex,

Please also put that kind of knowledge into the commit message upfront. No 
need to be as verbose as you're here (with code quotes, etc.) but stating 
that some platforms use higher IDs (e.g., Arm) would be pretty useful in 
answering the question why you're doing this change (which is one of the 
key points of describing your change).

-- 
 i.

> >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
> >
> 
>