Date: Tue, 1 Aug 2017 22:14:38 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Vikas Shivappa <vikas.shivappa@linux.intel.com>
cc: vikas.shivappa@intel.com, x86@kernel.org, linux-kernel@vger.kernel.org,
        hpa@zytor.com, peterz@infradead.org, ravi.v.shankar@intel.com,
        tony.luck@intel.com, fenghua.yu@intel.com, eranian@google.com,
        davidcc@google.com, ak@linux.intel.com, reinette.chatre@intel.com
Subject: Re: [PATCH 09/28] x86/intel_rdt/cqm: Add RMID(Resource monitoring
 ID) management
In-Reply-To: <1501017287-28083-10-git-send-email-vikas.shivappa@linux.intel.com>
Message-ID: <alpine.DEB.2.20.1708012207531.2682@nanos>
References: <1501017287-28083-1-git-send-email-vikas.shivappa@linux.intel.com> <1501017287-28083-10-git-send-email-vikas.shivappa@linux.intel.com>
User-Agent: Alpine 2.20 (DEB 67 2015-01-07)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1174
Lines: 26

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