linux-kernel - Re: [PATCH V4 1/1] x86/resctrl: Remove unappropriate references to cacheinfo in the resctrl subsystem.

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Message-ID: <ae018428-7a5c-49ec-bbc0-c93f1f8b965d@intel.com>
Date: Fri, 30 May 2025 10:58:18 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Qinyun Tan <qinyuntan@...ux.alibaba.com>, Tony Luck <tony.luck@...el.com>
CC: "H . Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>
Subject: Re: [PATCH V4 1/1] x86/resctrl: Remove unappropriate references to
 cacheinfo in the resctrl subsystem.

Hi Qinyun Tan,

On 5/30/25 10:50 AM, Qinyun Tan wrote:
> In the resctrl subsystem's Sub-NUMA Cluster (SNC) mode, the rdt_mon_domain
> structure representing a NUMA node relies on the cacheinfo interface
> (rdt_mon_domain::ci) to store L3 cache information (e.g., shared_cpu_map)
> for monitoring. The L3 cache information of a SNC NUMA node determines
> which domains are summed for the "top level" L3-scoped events.
> 
> rdt_mon_domain::ci is initialized using the first online CPU of a NUMA
> node. When this CPU goes offline, its shared_cpu_map is cleared to contain
> only the offline CPU itself. Subsequently, attempting to read counters
> via smp_call_on_cpu(offline_cpu) fails (and error ignored), returning
> zero values for "top-level events" without any error indication. Replace

nit: the solution description starts with the word "Replace" above. It
should be in a new paragraph to separate the solution description from the
problem description.

> the cacheinfo references in struct rdt_mon_domain and struct rmid_read
> with the cacheinfo ID (a unique identifier for the L3 cache).
> 
> rdt_domain_hdr::cpu_mask contains the online CPUs associated with that
> domain. When reading "top-level events", select a CPU from
> rdt_domain_hdr::cpu_mask and utilize its L3 shared_cpu_map to determine
> valid CPUs for reading RMID counter via the MSR interface.
> 
> Considering all CPUs associated with the L3 cache improves the chances
> of picking a housekeeping CPU on which the counter reading work can be
> queued, avoiding an unnecessary IPI.
> 
> Fixes: 328ea68874642 ("x86/resctrl: Prepare for new Sub-NUMA Cluster (SNC) monitor files")
> Signed-off-by: Qinyun Tan <qinyuntan@...ux.alibaba.com>
> Tested-by: Tony Luck <tony.luck@...el.com>
> Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
Reinette