Hi Reinette,
On 11/18/2024 4:07 PM, Reinette Chatre wrote:
Hi Babu,
On 11/18/24 11:04 AM, Moger, Babu wrote:
Hi Reinette,
On 11/15/24 18:00, Reinette Chatre wrote:
Hi Babu,
On 10/29/24 4:21 PM, Babu Moger wrote:
Introduce the interface file "mbm_assign_mode" to list monitor modes
supported.
The "mbm_cntr_assign" mode provides the option to assign a counter to
an RMID, event pair and monitor the bandwidth as long as it is assigned.
On AMD systems "mbm_cntr_assign" is backed by the ABMC (Assignable
Bandwidth Monitoring Counters) hardware feature and is enabled by default.
The "default" mode is the existing monitoring mode that works without the
explicit counter assignment, instead relying on dynamic counter assignment
by hardware that may result in hardware not dedicating a counter resulting
in monitoring data reads returning "Unavailable".
Provide an interface to display the monitor mode on the system.
$ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
[mbm_cntr_assign]
default
Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
---
...
I'm concerned that users with Intel platforms may want to use the "mbm_cntr_assign" mode
to make the event data "more predictable" and then be concerned when the mode does
not exist.
As an alternative, is it possible to know the number of hardware counters on AMD systems
without ABMC? I wonder if we could perhaps always expose num_mbm_cntrs as a way for
users to know if their platform may be impacted by this type of "unpredictability" (by comparing
num_mbm_cntrs to num_rmids).
There is some round about(or hacky) way to find that out number of RMIDs
that can be active.
Does this give consistent and accurate data? Is this something that can be added to resctrl?
(Reading your other message [1] it does not sound as though it can produce an accurate
number on boot.)
If not then it will be up to the documentation to be accurate.
+
+ AMD Platforms with ABMC (Assignable Bandwidth Monitoring Counters) feature
+ enable this mode by default so that counters remain assigned even when the
+ corresponding RMID is not in use by any processor.
+
+ "default":
+
+ In default mode resctrl assumes there is a hardware counter for each
+ event within every CTRL_MON and MON group. Reading mbm_total_bytes or
+ mbm_local_bytes may report 'Unavailable' if there is no counter associated
+ with that event.
If I understand correctly, on AMD platforms without ABMC the events only report
"Unavailable" if there is no counter assigned at the time of the query. If a counter
is unassigned and then reassigned then the event count will reset and the user
will get some data back but it may thus be unpredictable (to match earlier language).
Is this correct? Any AMD platform in "default" mode may thus be vulnerable to
"unpredictable" event counts (not just "Unavailable") ... this gets complicated
Yes. All the AMD systems without ABMC are affected by this problem.
because users should be steered to avoid "default" mode if mbm_assign_mode is
available, while not be made concerned to use "default" mode on Intel where
mbm_assign_mode is not available.
Can we add text to clarify this?
Please do.
I think we need to add text about AMD systems. How about this?
"default":
In default mode resctrl assumes there is a hardware counter for each
event within every CTRL_MON and MON group. On AMD systems with 16 more
monitoring groups, reading mbm_total_bytes or mbm_local_bytes may report
'Unavailable' if there is no counter associated with that event. It is
therefore recommended to use the 'mbm_cntr_assign' mode, if supported."
Reinette
[1] https://lore.kernel.org/all/35fc70fd-0281-4ac8-b32b-efa2f4516901@xxxxxxx/
--
- Babu Moger
