linux-kernel - Re: [PATCH 09/28] x86/intel_rdt/cqm: Add RMID(Resource monitoring ID) management

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Date:   Tue, 1 Aug 2017 22:14:38 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>
cc:     vikas.shivappa@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, peterz@...radead.org,
        ravi.v.shankar@...el.com, tony.luck@...el.com,
        fenghua.yu@...el.com, eranian@...gle.com, davidcc@...gle.com,
        ak@...ux.intel.com, reinette.chatre@...el.com
Subject: Re: [PATCH 09/28] x86/intel_rdt/cqm: Add RMID(Resource monitoring
 ID) management

On Tue, 25 Jul 2017, Vikas Shivappa wrote:
> Hardware uses RMID(Resource monitoring ID) to keep track of each of the
> RDT events associated with tasks. The number of RMIDs is dependent on
> the SKU and is enumerated via CPUID. We add support to manage the RMIDs
> which include managing the RMID allocation and reading LLC occupancy
> for an RMID.
> 
> RMID allocation is managed by keeping a free list which is initialized
> to all available RMIDs except for RMID 0 which is always reserved for
> root group. RMIDs goto a limbo list once they are
> freed since the RMIDs are still tagged to cache lines of the tasks which
> were using them - thereby still having some occupancy. They continue to
> be in limbo list until the occupancy < threshold_occupancy. The
> threshold_occupancy is a user configurable value.
> OS uses IA32_QM_CTR MSR to read the occupancy associated with an RMID
> after programming the IA32_EVENTSEL MSR with the RMID.
> 
> [Tony: Improved limbo search]

The search is smarter, but the smp function call problem per se
persists. It's still more than 100us worstcase on a BDW box and it's going
to be worse with the next generation.

Thanks,

	tglx