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Received: from DM4PR12MB5230.namprd12.prod.outlook.com (2603:10b6:5:399::11) by CH1PR12MB9622.namprd12.prod.outlook.com (2603:10b6:610:2b2::17) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.181.16; Thu, 9 Jul 2026 16:43:50 +0000 Received: from DM4PR12MB5230.namprd12.prod.outlook.com ([fe80::6e87:1bde:1853:3b73]) by DM4PR12MB5230.namprd12.prod.outlook.com ([fe80::6e87:1bde:1853:3b73%5]) with mapi id 15.21.0181.014; Thu, 9 Jul 2026 16:43:50 +0000 Message-ID: <4ea54158-915b-4778-a5f4-80e0e6e71c1d@nvidia.com> Date: Thu, 9 Jul 2026 09:43:47 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 5/5] arm64: mpam: Add memory bandwidth usage (MBWU) documentation To: Ben Horgan Cc: amitsinght@marvell.com, baisheng.gao@unisoc.com, baolin.wang@linux.alibaba.com, carl@os.amperecomputing.com, dave.martin@arm.com, david@kernel.org, dfustini@baylibre.com, gshan@redhat.com, james.morse@arm.com, jic23@kernel.org, kobak@nvidia.com, lcherian@marvell.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, peternewman@google.com, punit.agrawal@oss.qualcomm.com, quic_jiles@quicinc.com, reinette.chatre@intel.com, rohit.mathew@arm.com, scott@os.amperecomputing.com, sdonthineni@nvidia.com, tan.shaopeng@fujitsu.com, xhao@linux.alibaba.com, zengheng4@huawei.com, x86@kernel.org, leitao@kernel.org, kas@kernel.org, puranjay@kernel.org, usama.arif@linux.dev, kernel-team@meta.com, Shaopeng Tan References: <20260707162440.12132-1-ben.horgan@arm.com> <20260707162440.12132-6-ben.horgan@arm.com> Content-Language: en-US From: Fenghua Yu In-Reply-To: <20260707162440.12132-6-ben.horgan@arm.com> Content-Type: text/plain; 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Add some documentation so the user knows what to > expect. > > Co-developed-by: James Morse > Signed-off-by: James Morse > Signed-off-by: Ben Horgan > Reviewed-by: Shaopeng Tan Reviewed-by: Fenghua Yu Thanks. -Fenghua > --- > Changes since v4: > Add paragraph about the future. > --- > Documentation/arch/arm64/mpam.rst | 22 ++++++++++++++++++++++ > 1 file changed, 22 insertions(+) > > diff --git a/Documentation/arch/arm64/mpam.rst b/Documentation/arch/arm64/mpam.rst > index 570f51a8d4eb..67fe515ed501 100644 > --- a/Documentation/arch/arm64/mpam.rst > +++ b/Documentation/arch/arm64/mpam.rst > @@ -65,6 +65,28 @@ The supported features are: > there is at least one CSU monitor on each MSC that makes up the L3 group. > Exposing CSU counters from other caches or devices is not supported. > > +* Memory Bandwidth Usage (MBWU) on or after the L3 cache. resctrl uses the > + L3 cache-id to identify where the memory bandwidth is measured. For this > + reason the platform must have an L3 cache with cache-id's supplied by > + firmware. (The platform doesn't need to support MPAM.) > + > + Memory bandwidth monitoring makes use of MBWU monitors in each MSC that > + makes up the L3 group. If the memory bandwidth monitoring is on the memory > + rather than the L3 then there must be a single global L3 as otherwise it > + is unknown which L3 the traffic came from. > + > + To expose 'mbm_total_bytes', the topology of the group of MSC chosen must > + match the topology of the L3 cache so that the cache-id's can be > + repainted. For example: Platforms with Memory bandwidth monitors on > + CPU-less NUMA nodes cannot expose 'mbm_total_bytes' as these nodes do not > + have a corresponding L3 cache. 'mbm_local_bytes' is not exposed as MPAM > + cannot distinguish local traffic from global traffic. > + > + All these restrictions based on L3 cache are due to resctrl, currently, only > + supporting monitoring at the L3 scope. It is expected that going forward more > + MBWU monitors can be exposed to the user after support for more monitoring > + scopes is added to resctrl. > + > Reporting Bugs > ============== > If you are not seeing the counters or controls you expect please share the