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

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Date: Mon, 11 Mar 2024 09:24:00 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Rex Nie <rex.nie@...uarmicro.com>
CC: <james.morse@....com>, <fenghua.yu@...el.com>,
	<reinette.chatre@...el.com>, <linux-kernel@...r.kernel.org>, Liming Wu
	<liming.wu@...uar.com>
Subject: Re: [PATCH] fs/resctrl: fix domid loss precision issue

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.

>/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?

>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@...uarmicro.com>
>Signed-off-by: Liming Wu <liming.wu@...uar.com>
>---
> 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