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arm64: mpam: Add memory bandwidth usage (MBWU) documentation

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Memory bandwidth monitoring make uses of MBWU monitors and is now exposed
to the user via resctrl. Add some documentation so the user knows what to
expect.

Co-developed-by: James Morse <james.morse@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Ben Horgan 2026-07-31 18:52:42 +01:00 committed by Will Deacon
parent a5ff6ef2d7
commit 6b4ee75215
1 changed files with 22 additions and 0 deletions
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Documentation/arch/arm64/mpam.rst
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@ -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
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If you are not seeing the counters or controls you expect please share the