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Correct. > > Why limit it to one CTRL_MON group and why not support it for MON groups? There can be only one PLZA configuration in a system. The values in the MSR_IA32_PQR_PLZA_ASSOC register (RMID, RMID_EN, CLOSID, CLOSID_EN) must be identical across all logical processors. The only field that may differ is PLZA_EN. I was initially unsure which RMID should be used when PLZA is enabled on MON groups. After re-evaluating, enabling PLZA on MON groups is still feasible: 1. Only one group in the system can have PLZA enabled. 2. If PLZA is enabled on CTRL_MON group then we cannot enable PLZA on MON group. 3. If PLZA is enabled on the CTRL_MON group, then the CLOSID and RMID of the CTRL_MON group can be written. 4. If PLZA is enabled on a MON group, then the CLOSID of the CTRL_MON group can be used, while the RMID of the MON group can be written. I am thinking this approach should work. > > Limiting it to a single CTRL group seems restrictive in a few ways: > 1) It requires that the "PLZA" group has a dedicated CLOSID. This reduces the > number of use cases that can be supported. Consider, for example, an existing > "high priority" resource group and a "low priority" resource group. The user may > just want to let the tasks in the "low priority" resource group run as "high priority" > when in CPL0. This of course may depend on what resources are allocated, for example > cache may need more care, but if, for example, user is only interested in memory > bandwidth allocation this seems a reasonable use case? > 2) Similar to what Tony [1] mentioned this does not enable what the hardware is > capable of in terms of number of different control groups/CLOSID that can be > assigned to MSR_IA32_PQR_PLZA_ASSOC. Why limit PLZA to one CLOSID? > 3) The feature seems to support RMID in MSR_IA32_PQR_PLZA_ASSOC similar to > MSR_IA32_PQR_ASSOC. With this, it should be possible for user space to, for > example, create a resource group that contains tasks of interest and create > a monitor group within it that monitors all tasks' bandwidth usage when in CPL0. > This will give user space better insight into system behavior and from what I can > tell is supported by the feature but not enabled? Yes, as long as PLZA is enabled on only one group in the entire system > >>> >>>> 2) It can't be the root/default group >>> >>> This is something I added to keep the default group in a un-disturbed, > > Why was this needed? > With the new approach mentioned about we can enable in default group also. >>> >>>> 3) It can't have sub monitor groups > > Why not? Ditto. With the new approach mentioned about we can enable in default group also. > >>>> 4) It can't be pseudo-locked >>> >>> Yes. >>> >>>> >>>> Would a potential use case involve putting *all* tasks into the PLZA group? That >>>> would avoid any additional context switch overhead as the PLZA MSR would never >>>> need to change. >>> >>> Yes. That can be one use case. >>> >>>> >>>> If that is the case, maybe for the PLZA group we should allow user to >>>> do: >>>> >>>> # echo '*' > tasks > > Dedicating a resource group to "PLZA" seems restrictive while also adding many > complications since this designation makes resource group behave differently and > thus the files need to get extra "treatments" to handle this "PLZA" designation. > > I am wondering if it will not be simpler to introduce just one new file, for example > "tasks_cpl0" in both CTRL_MON and MON groups. When user space writes a task ID to the > file it "enables" PLZA for this task and that group's CLOSID and RMID is the associated > task's "PLZA" CLOSID and RMID. This gives user space the flexibility to use the same > resource group to manage user space and kernel space allocations while also supporting > various monitoring use cases. This still supports the "dedicate a resource group to PLZA" > use case where user space can create a new resource group with certain allocations but the > "tasks" file will be empty and "tasks_cpl0" contains the tasks needing to run with > the resource group's allocations when in CPL0. Yes. We should be able do that. We need both tasks_cpl0 and cpus_cpl0. We need make sure only one group can configured in the system and not allow in other groups when it is already enabled. Thanks Babu > > Reinette > > [1] https://lore.kernel.org/lkml/aXpgragcLS2L8ROe@agluck-desk3/ >