[PATCH v5 0/2] x86,fs/resctrl: Keep default MBM mode at boot and fix ABMC programming

[PATCH v5 0/2] x86,fs/resctrl: Keep default MBM mode at boot and fix ABMC programming

[Babu Moger]

Hi All,

This series restores default MBM assignment mode at boot, and fixes ABMC
programming for more than 32 counters.

Commit 0f1576e43adc ("x86/resctrl: Configure mbm_event mode if supported")
enabled ABMC at boot. That breaks the pqos tool from intel-cmt-cat [1],
which treats a non-numeric event read as zero bandwidth. pqos creates 16
or more groups and uses two counters per group. On a platform with 32
counters per domain, that consumes the pool, so further groups read
"Unassigned" and pqos reports 0 MB/s.

Leaving the mode at default does not make pqos correct. pqos still
treats "Unavailable" as zero. Once the default-mode counter pool is
exceeded, hardware can re-allocate a counter between reads and pqos can
report that as a wraparound. That shows up only after the pool is
exceeded. Default mode uses one counter per monitoring group. On existing
AMD platforms that pool is 64 counters and may be larger on newer
hardware, so common deployments stay within it. mbm_event mode uses two
counters per group, so the same 32 counters cover only 16 groups.

mbm_event remains available as an opt-in. Users that need stable
measurements beyond the default-mode pool should enable it and rotate
assignments. The limitation of default mode is documented.

Patch 1: widen cntr_id in L3_QOS_ABMC_CFG from 5 bits to 12 bits, and
limit the enumerated counter count to 4096, which is what that field can
encode. The APM [2] at [3] will be updated for the expanded cntr_id field.

Patch 2: leave mbm_assign_mode as "default" at boot. Counters are not
assigned automatically. So, update resctrl documentation to replect that.

Based on tip/master:
  6b27d3127c05 Merge branch into tip/master: 'x86/tdx'

[1] https://github.com/intel/intel-cmt-cat/issues/311
[2] AMD64 Architecture Programmer's Manual Volume 2: System Programming,
    Publication #24593, Revision 3.41, Section 19.3.3.3 "Assignable
    Bandwidth Monitoring (ABMC)"
[3] https://bugzilla.kernel.org/show_bug.cgi?id=206537

v5:
 - Dropped patch 2 from v4, as auto-assignment is not a requirement
   and should be left to the user.
 - Patch 1: Added Reviewed-by tag from Reinette.
 - Patch 2: Fixed a few typos.
            Auto-assignment is not a requirement based on our discussion.
            Updated the resctrl documentation to reflect this.

v4:
 - Patch 1: Updated changelog(from Reinette's input).
 - Patch 2: Changelog update (Reinette's input).
            Function comment update for rdtgroup_assign_cntrs().
            Tag order update.
 - Patch 3: Introduced mbm_event counter pool and default active counter pool.
            Removed duplicate texts in resctrl.rst.
            Re-arranged the tag order.
v3:
- Patch 1: drop the 32-bit u64 change and the bw_src widening. Cap the
  enumerated counter count at 4096 instead of truncating the CPUID field.
- Patch 2: clean up comments. Add Reported-by, Closes, and Cc: stable.
- Patch 3: document one counter per group in default mode and two counters
  per group in mbm_event mode. Describe the default-mode pool and the pqos
  wraparound once that pool is exceeded.

v2:
- Added patch 2 to address Sashiko's comment regarding the documentation
  issue. In fact, it exposed a real issue. When switching to mbm_event mode,
  existing monitoring groups should be assigned counters whenever counters
  are available. This provides a smooth transition between modes and aligns
  the behavior with the existing auto-assignment mechanism.
  https://sashiko.dev/#/patchset/8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger%40amd.com

- Combine the two v1 patches and add patch 2 (assign existing groups
  on mbm_event switch; Sashiko review).
- Keep boot-default separate from the encoding/truncation fixes.
- Document the default-mode counter-pool limitation.

Previous versions:

v1: https://lore.kernel.org/lkml/980f39d3a0e0d9f73925e362f835aeef070a1bc5.1784322818.git.babu.moger@amd.com/
v2: https://lore.kernel.org/lkml/cover.1788545152.git.babu.moger@amd.com/
v3: https://lore.kernel.org/lkml/cover.1790976400.git.babu.moger@amd.com/
v4: https://lore.kernel.org/lkml/cover.1791589412.git.babu.moger@amd.com/

Thanks
Babu Moger


Babu Moger (2):
  x86/resctrl: Fix ABMC counter programming
  x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot

 Documentation/filesystems/resctrl.rst  | 101 +++++++++++++++----------
 arch/x86/kernel/cpu/resctrl/internal.h |   4 +-
 arch/x86/kernel/cpu/resctrl/monitor.c  |   4 +-
 3 files changed, 67 insertions(+), 42 deletions(-)

-- 
2.43.0

[PATCH v5 1/2] x86/resctrl: Fix ABMC counter programming

[Babu Moger]

AMD's Assignable Bandwidth Monitoring Counters (ABMC) are configured via
MSR_IA32_L3_QOS_ABMC_CFG; MSR_IA32_L3_QOS_ABMC_CFG.cntr_id selects the
counter the configuration applies to. The number of counters a platform
supports (the number of possible values written to
MSR_IA32_L3_QOS_ABMC_CFG.cntr_id) is enumerated separately via CPUID.

On platforms that enumerate more than 32 counters, the current 5-bit
encoding truncates the counter ID and misprograms ABMC.

The AMD64 Architecture Programmer's Manual [1], available from [2], has
been updated to widen MSR_IA32_L3_QOS_ABMC_CFG.cntr_id from 5 bits to 12
bits (the published revision 3.41 does not yet reflect this; a future
revision will). The CPUID enumeration reports the maximum counter ID in a
16-bit field and may therefore report more counters than a 12-bit
MSR_IA32_L3_QOS_ABMC_CFG.cntr_id can address.

Widen MSR_IA32_L3_QOS_ABMC_CFG.cntr_id to 12 bits to match the
architecture. Cap the enumerated counter count at BIT(12) so every counter
ID used by resctrl can be written to MSR_IA32_L3_QOS_ABMC_CFG.cntr_id
without truncation.

[1] AMD64 Architecture Programmer's Manual Volume 2: System Programming,
    Publication #24593, Revision 3.41, Section 19.3.3.3 "Assignable
    Bandwidth Monitoring (ABMC)"

Fixes: 84ecefb76674 ("x86/resctrl: Add data structures and definitions for ABMC assignment")
Signed-off-by: Babu Moger <babu.moger@amd.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Cc: stable@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537 # [2]
---
v5: Added Reviewed-by:(Reinette).

v4: Changelog update(from Reinette's input).

v3: Dropped the fix for truncation on 32-bit x86.
    Removed the change bw_src field(RMID) width to 15 bits.
    Added new check to limit the number of counters to 12 bits.

v2: Moved the link tag to the last.

v1: https://lore.kernel.org/lkml/980f39d3a0e0d9f73925e362f835aeef070a1bc5.1784322818.git.babu.moger@amd.com/
---
 arch/x86/kernel/cpu/resctrl/internal.h | 4 ++--
 arch/x86/kernel/cpu/resctrl/monitor.c  | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
index e3cfa0c10e92..ffd74a68671b 100644
--- a/arch/x86/kernel/cpu/resctrl/internal.h
+++ b/arch/x86/kernel/cpu/resctrl/internal.h
@@ -214,8 +214,8 @@ union l3_qos_abmc_cfg {
 			      bw_src   :12,
 			      reserved1: 3,
 			      is_clos  : 1,
-			      cntr_id  : 5,
-			      reserved : 9,
+			      cntr_id  :12,
+			      reserved : 2,
 			      cntr_en  : 1,
 			      cfg_en   : 1;
 	} split;
diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 3838e0a13d36..5d0d3b18f9b8 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -470,7 +470,8 @@ int __init rdt_get_l3_mon_config(struct rdt_resource *r)
 		r->mon.mbm_cntr_assignable = true;
 		r->mon.mbm_cntr_configurable = true;
 		cpuid_count(0x80000020, 5, &eax, &ebx, &ecx, &edx);
-		r->mon.num_mbm_cntrs = (ebx & GENMASK(15, 0)) + 1;
+		/* cntr_id is 12 bits and can only encode 4096 counters. */
+		r->mon.num_mbm_cntrs = min((ebx & GENMASK(15, 0)) + 1, BIT(12));
 		hw_res->mbm_cntr_assign_enabled = true;
 	}
 
-- 
2.43.0

[PATCH v5 2/2] x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot

[Babu Moger]

The mbm_event mode assigns hardware MBM counters to RMID/event pairs.
It is intended for deployments that need to manage counter assignment
on platforms where the number of monitoring groups exceeds the available
hardware counters. Hardware counters are scarce on these platforms, so
mbm_event mode suits workflows that monitor a subset of groups at a time
and rotate assignments as needed.

resctrl enables the mbm_event mode by default on hardware that supports
it. This breaks the pqos tool [1], which assumes the historical default
mode. It creates 16 or more monitoring groups and uses two counters per
group. On a platform with 32 counters per domain, that exhausts the
mbm_event counters pool, causing additional groups to read "Unassigned".
pqos interprets a non-numeric event read as zero bandwidth and reports 0
MB/s for those groups:

  $sudo pqos -m all:0-4
  CORE         IPC      MISSES     LLC[KB]   MBL[MB/s]   MBR[MB/s]
     0        0.69         16k       192.0         0.0         0.0
     1        0.40          3k         0.0         0.0         0.0
     2        0.44          1k        64.0         0.0         0.0
     3        0.39          1k        64.0         0.0         0.0
     4        0.43          1k         0.0         0.0         0.0

Leave mbm_assign_mode in default mode during initialization. Default mode
uses one counter per monitoring group. On existing AMD platforms, the
default active counters pool is 64 and may be larger on newer hardware.
The active counters pool is the number of counters that the hardware can
actively track in default mode.

Deployments within the active counters pool receive accurate bandwidth
measurements in the default mode. Users can create more monitoring groups
(for example, 4096 on current hardware) than there are active counters in
the pool. Beyond that pool, readings may be misleading or report as
"Unavailable", with no user-visible indication.

Default mode is still limited once the number of monitoring groups exceeds
that active counters pool. After hardware re-allocates a counter, a read
may return "Unavailable". pqos still treats "Unavailable" as zero.
Successive reads can be a count, "Unavailable", then another count. pqos
treats those jumps as wraparound and can report an inconsistent rate:

  $sudo pqos -m all:0-4
  CORE     IPC      MISSES     LLC[KB]   MBL[MB/s]   MBR[MB/s]
  0        0.76         60k        32.0         0.0         0.0
  1        0.45          1k        32.0         0.0         0.0
  2        1.57        107k        64.0         0.0 17592186044184.9
  3        1.62        276k      4928.0         3.2         0.5

Users that need stable measurements beyond the active counters pool should
use mbm_event mode and rotate assignments as needed. Enable it with:

  $echo mbm_event > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode

Fixes: 0f1576e43adc ("x86/resctrl: Configure mbm_event mode if supported")
Closes: https://github.com/intel/intel-cmt-cat/issues/311
Signed-off-by: Babu Moger <babu.moger@amd.com>
Cc: stable@vger.kernel.org
Link: https://github.com/intel/intel-cmt-cat # [1]
---
v5: Dropped the patch 2 from v4. As auto-assign is not a requirement when
    mode is switched to mbm_event, the examples needed some changes.
    Updated resctrl.rst to take care of that.
    Few minor changelog update.

v4: Introduced mbm_event counter pool and default active counter pool.
    Removed duplicate texts in resctrl.rst.
    Re-arranged the tag order.

v3: Added pqos output to show the problem.
    More changelog to detail the issue.
    Updated the resctrl.rst to mention changes.

v2:
  Added documentation describing the known issue with the default mode.
  Will add cc to stable once we have all the things in order.
  Let me know if I missed anything.

v1:
  https://lore.kernel.org/lkml/8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger@amd.com/
---
 Documentation/filesystems/resctrl.rst | 101 ++++++++++++++++----------
 arch/x86/kernel/cpu/resctrl/monitor.c |   1 -
 2 files changed, 63 insertions(+), 39 deletions(-)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index b52795e03303..fc3ced99e3b7 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -354,8 +354,8 @@ with the following files:
 	::
 
 	  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
-	  [mbm_event]
-	  default
+	  [default]
+	  mbm_event
 
 	"mbm_event":
 
@@ -373,20 +373,35 @@ with the following files:
 	Moving to mbm_event counter assignment mode requires users to assign the counters
 	to the events. Otherwise, the MBM event counters will return 'Unassigned' when read.
 
-	The mode is beneficial for AMD platforms that support more CTRL_MON
-	and MON groups than available hardware counters. By default, this
-	feature is enabled on AMD platforms with the ABMC (Assignable Bandwidth
-	Monitoring Counters) capability, ensuring counters remain assigned even
-	when the corresponding RMID is not actively used by any processor.
+	It is intended for deployments that need to manage counter assignment on
+	platforms where the number of monitoring groups exceeds the available
+	hardware counters. Hardware counters are scarce on these platforms, so
+	mbm_event mode suits workflows that monitor a subset of groups at a time and
+	rotate assignments as needed. The mbm_event mode ensures counters remain
+	assigned even when the corresponding RMID is not actively monitored.
 
 	"default":
 
 	In default mode, resctrl assumes there is a hardware counter for each
-	event within every CTRL_MON and MON group. On AMD platforms, it is
-	recommended to use the mbm_event mode, if supported, to prevent reset of MBM
-	events between reads resulting from hardware re-allocating counters. This can
-	result in misleading values or display "Unavailable" if no counter is assigned
-	to the event.
+	event within every CTRL_MON and MON group. This mode is enabled by default.
+
+	On AMD platforms that support more CTRL_MON and MON groups than hardware
+	counters, hardware dynamically shares a smaller pool of counters among
+	RMIDs. One counter from this pool is used per monitoring group and counts
+	every MBM event of that group's RMID. The size of the pool is not
+	enumerated to software (unlike "num_mbm_cntrs"), and "num_rmids" may be
+	much larger. For example, 64 counters in this pool monitor 64 groups.
+	Newer hardware may provide a larger pool.
+
+	While the number of monitoring groups does not exceed that pool, a counter
+	stays attached to each RMID and readings remain accurate. Creating more
+	groups than the pool can cause hardware to re-allocate those counters
+	between successive reads of an event. Bandwidth values may then be
+	misleading, or a read may return "Unavailable" if no counter is allocated
+	to the RMID. There is no user-visible indication when this begins.
+	Users who need stable readings beyond that pool should switch to mbm_event
+	mode, if supported, and assign counters to the groups of interest
+	(rotating assignments as needed).
 
 	* To enable "mbm_event" counter assignment mode:
 	  ::
@@ -1784,62 +1799,72 @@ View the llc occupancy snapshot::
 Examples on working with mbm_assign_mode
 ========================================
 
-a. Check if MBM counter assignment mode is supported.
+a. Check if MBM counter assignment mode is supported. The boot mode is "default".
 ::
 
-  # mount -t resctrl resctrl /sys/fs/resctrl/
+  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
+  [default]
+  mbm_event
+
+The "default" mode is enabled. "mbm_event" is supported but not enabled.
 
+b. Enable "mbm_event" mode. Switching the mbm_assign_mode may
+reset all the MBM counters (and thus all MBM events) of all the resctrl groups.
+::
+
+  # echo "mbm_event" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
   # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
   [mbm_event]
   default
 
-The "mbm_event" mode is detected and enabled.
-
-b. Check how many assignable counters are supported.
+c. Check how many assignable counters are supported.
 ::
 
   # cat /sys/fs/resctrl/info/L3_MON/num_mbm_cntrs
   0=32;1=32
 
-c. Check how many assignable counters are available for assignment in each domain.
+d. Check how many assignable counters are available for assignment in each domain.
 ::
 
   # cat /sys/fs/resctrl/info/L3_MON/available_mbm_cntrs
-  0=30;1=30
+  0=32;1=32
 
-d. To list the default group's assign states.
+e. To list the default group's assign states. Counters are not assigned
+automatically. They are unassigned when the mode is switched to "mbm_event".
 ::
 
   # cat /sys/fs/resctrl/mbm_L3_assignments
-  mbm_total_bytes:0=e;1=e
-  mbm_local_bytes:0=e;1=e
+  mbm_total_bytes:0=_;1=_
+  mbm_local_bytes:0=_;1=_
 
-e.  To unassign the counter associated with the mbm_total_bytes event on domain 0.
+f. To assign the counter associated with the mbm_total_bytes event on domain 0
+in exclusive mode.
 ::
 
-  # echo "mbm_total_bytes:0=_" > /sys/fs/resctrl/mbm_L3_assignments
+  # echo "mbm_total_bytes:0=e" > /sys/fs/resctrl/mbm_L3_assignments
   # cat /sys/fs/resctrl/mbm_L3_assignments
-  mbm_total_bytes:0=_;1=e
-  mbm_local_bytes:0=e;1=e
+  mbm_total_bytes:0=e;1=_
+  mbm_local_bytes:0=_;1=_
 
-f. To unassign the counter associated with the mbm_total_bytes event on all domains.
+g. To unassign the counter associated with the mbm_total_bytes event on all domains.
 ::
 
   # echo "mbm_total_bytes:*=_" > /sys/fs/resctrl/mbm_L3_assignments
-  # cat /sys/fs/resctrl/mbm_L3_assignment
+  # cat /sys/fs/resctrl/mbm_L3_assignments
   mbm_total_bytes:0=_;1=_
-  mbm_local_bytes:0=e;1=e
+  mbm_local_bytes:0=_;1=_
 
-g. To assign a counter associated with the mbm_total_bytes event on all domains in
-exclusive mode.
+h. To assign a counter associated with the mbm_total_bytes and mbm_local_bytes
+events on all domains in exclusive mode.
 ::
 
   # echo "mbm_total_bytes:*=e" > /sys/fs/resctrl/mbm_L3_assignments
+  # echo "mbm_local_bytes:*=e" > /sys/fs/resctrl/mbm_L3_assignments
   # cat /sys/fs/resctrl/mbm_L3_assignments
   mbm_total_bytes:0=e;1=e
   mbm_local_bytes:0=e;1=e
 
-h. Read the events mbm_total_bytes and mbm_local_bytes of the default group. There is
+i. Read the events mbm_total_bytes and mbm_local_bytes of the default group. There is
 no change in reading the events with the assignment.
 ::
 
@@ -1852,7 +1877,7 @@ no change in reading the events with the assignment.
   # cat /sys/fs/resctrl/mon_data/mon_L3_01/mbm_local_bytes
   121212144
 
-i. Check the event configurations.
+j. Check the event configurations.
 ::
 
   # cat /sys/fs/resctrl/info/L3_MON/event_configs/mbm_total_bytes/event_filter
@@ -1862,7 +1887,7 @@ i. Check the event configurations.
   # cat /sys/fs/resctrl/info/L3_MON/event_configs/mbm_local_bytes/event_filter
   local_reads,local_non_temporal_writes,local_reads_slow_memory
 
-j. Change the event configuration for mbm_local_bytes.
+k. Change the event configuration for mbm_local_bytes.
 ::
 
   # echo "local_reads, local_non_temporal_writes, local_reads_slow_memory, remote_reads" >
@@ -1871,7 +1896,7 @@ j. Change the event configuration for mbm_local_bytes.
   # cat /sys/fs/resctrl/info/L3_MON/event_configs/mbm_local_bytes/event_filter
   local_reads,local_non_temporal_writes,local_reads_slow_memory,remote_reads
 
-k. Now read the local events again. The first read may come back with "Unavailable"
+l. Now read the local events again. The first read may come back with "Unavailable"
 status. The subsequent read of mbm_local_bytes will display the current value.
 ::
 
@@ -1884,17 +1909,17 @@ status. The subsequent read of mbm_local_bytes will display the current value.
   # cat /sys/fs/resctrl/mon_data/mon_L3_01/mbm_local_bytes
   1566565
 
-l. Users have the option to go back to 'default' mbm_assign_mode if required. This can be
+m. Users have the option to go back to 'default' mbm_assign_mode if required. This can be
 done using the following command. Note that switching the mbm_assign_mode may reset all
 the MBM counters (and thus all MBM events) of all the resctrl groups.
 ::
 
   # echo "default" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
   # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
-  mbm_event
   [default]
+  mbm_event
 
-m. Unmount the resctrl filesystem.
+n. Unmount the resctrl filesystem.
 ::
 
   # umount /sys/fs/resctrl/
diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 5d0d3b18f9b8..a6a9090c4808 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -472,7 +472,6 @@ int __init rdt_get_l3_mon_config(struct rdt_resource *r)
 		cpuid_count(0x80000020, 5, &eax, &ebx, &ecx, &edx);
 		/* cntr_id is 12 bits and can only encode 4096 counters. */
 		r->mon.num_mbm_cntrs = min((ebx & GENMASK(15, 0)) + 1, BIT(12));
-		hw_res->mbm_cntr_assign_enabled = true;
 	}
 
 	r->mon_capable = true;
-- 
2.43.0