答复: [PATCH] fs/resctrl: fix domid loss precision issue

[Rex Nie]

Hello,
	Please kindly check my inline reply. Thanks.
Best regards
Rex Nie

> -----邮件原件-----
> 发件人: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
> 发送时间: 2024年3月11日 16:24
> 收件人: Rex Nie <rex.nie@xxxxxxxxxxxxxxx>
> 抄送: james.morse@xxxxxxx; fenghua.yu@xxxxxxxxx;
> reinette.chatre@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Liming Wu
> <liming.wu@xxxxxxxxxx>
> 主题: Re: [PATCH] fs/resctrl: fix domid loss precision issue
> 
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> 
> Hello,
> 
> On 2024-03-11 at 14:48:22 +0800, Rex Nie wrote:
> >Below statement from mkdir_mondata_subdir function will loss precision,
> >because it assigns int to 14 bits bitfield.
> >       priv.u.domid = d->id;
> >
> >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

> >/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

> >cat: read error: No such file or directory
> >
> >This is the call trace when cat llc_occupancy:
> >rdtgroup_mondata_show()
> >       domid = md.u.domid
> >       d = resctrl_arch_find_domain(r, domid)
> >
> >d is null here because of lossing precision
> >
> >Signed-off-by: Rex Nie <rex.nie@xxxxxxxxxxxxxxx>
> >Signed-off-by: Liming Wu <liming.wu@xxxxxxxxxx>
> >---
> > fs/resctrl/internal.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h index
> >7a6f46b4edd0..096317610949 100644
> >--- a/fs/resctrl/internal.h
> >+++ b/fs/resctrl/internal.h
> >@@ -94,7 +94,7 @@ union mon_data_bits {
> >       struct {
> >               unsigned int rid                : 10;
> >               enum resctrl_event_id evtid     : 8;
> >-              unsigned int domid              : 14;
> >+              u32                             domid;
> >       } u;
> > };
> >
> >--
> >2.34.1
> >
> 
> --
> Kind regards
> Maciej Wieczór-Retman