Re: [RESEND PATCH v4 04/15] fs/resctrl: Introduce kernel mode (kmode) data structures

[Babu Moger]

Hi Reinette,

On 8/13/26 13:18, Reinette Chatre wrote:
> Hi Babu,
>
> On 8/13/26 10:12 AM, Babu Moger wrote:
> > Hi Reinette,
> >
> > On 8/13/26 10:55, Reinette Chatre wrote:
> > > Hi Babu,
> > >
> > > On 8/13/26 8:17 AM, Babu Moger wrote:
> > > > On 8/12/26 18:28, Reinette Chatre wrote:
> > >
> > > > > Please always keep in mind all the requirements and use cases we learned about
> > > > > during and after RFC v1 of this work.
> > > > >
> > > > > For example, we already know that "per group" assignment is something resctrl
> > > > > needs to be ready for. Consider the example in
> > > > > https://lore.kernel.org/lkml/aYyxAPdTFejzsE42@xxxxxxxxxxxxxxx/
> > > > >
> > > > > There may even be "per task" assignment in the future.
> > > >
> > > > Yes. That is correct.
> > > >
> > > > > Constraining this feature to PLZA will make it harder to enable the capabilities
> > > > > that we know resctrl need to support in the future.
> > > > >
> > > > > This is how we originally landed on the "global" assignment distinction
> > > > > (https://lore.kernel.org/lkml/2ab556af-095b-422b-9396-f845c6fd0342@xxxxxxxxx/)
> > > > > "global assignment" should be kept or replaced with a solution that continues to
> > > > > prepare resctrl for these other capabilities.
> > > >
> > > > Yes. Makes sense.
> > > >
> > > > > I am not able to see how resctrl could support "per group" assignment with the
> > > > > interface you propose above. If I am missing this, please highlight the solution.
> > > > >
> > > > > resctrl may need to explicitly split kernel mode from kernel mode properties. For
> > > > > example, below shows an "assign_global_enable_per_cpu" as the kernel mode, now with three
> > > > > properties:
> > > > > - "ctrl" - could be "assign" or "inherit"
> > > > > - "mon" - could be "assign" or "inherit"
> > > > > - "group" - required if "ctrl" or "mon" is set to "assign"
> > > >
> > > > ok.
> > > >
> > > >
> > > > >       # cat info/kernel_mode
> > > > >       [inherit]
> > > > >       assign_global_enable_per_cpu:ctrl=assign;mon=assign;group=uninitialized
> > > >
> > > > Shouldn't this be like below when default is inherit?
> > > >
> > > > # cat info/kernel_mode
> > > > [inherit]
> > > > assign_global_enable_per_cpu
> > >
> > > I think it will be useful to let the interface:
> > >    (a) show to user space which properties are available for each supported mode, and
> > >    (b) show user space what the default value of those properties will be if they
> > >        are not set when the associated mode is enabled.
> >
> > The "inherit" is the default property if it is not explicitly set. Right?
>
> "property" is different from "kernel mode"
>
> Each "kernel mode" can have zero or more properties.
> "inherit" is the default "kernel mode", but it could have a better/more descriptive name.
> For example, "inherit_from_user" or ...?

Or "user_inherit_kernel" ?

> > # cat info/kernel_mode
> > [inherit]
> > assign_global_enable_per_cpu:ctrl=inherit;mon=inherit;group=uninitialized
> >
> >
> > If the intention is to display all supported values for each
> > property, then we should do so consistently for all properties. For
> > example:
>
> No. The intention is not to display all supported values for properties of the
> different kernel modes. Just display which properties are supported and what value resctrl
> would use if the user enables that mode without providing a value for a particular
> property. There may be properties that could have values for which it will
> be difficult to provide all supported values.
>
> For example, if "kernel_mode" contains:
>        # cat info/kernel_mode
>        [inherit]
>        assign_global_enable_per_cpu:ctrl=assign;mon=assign;group=//
>
> Then user space knows that if they enable "assign_global_enable_per_cpu" kernel mode
> without providing any properties then all kernel work will use the default resource
> group's allocation and monitoring. If that is not what user space wants then they
> can change the value of only the properties they need to change.

That sounds reasonable.

When the group is bound to the global mode,

# cat info/kernel_mode
  inherit
  [assign_global_enable_per_cpu:ctrl=assign;group=ctrl1//]

> > # cat info/kernel_mode
> > [inherit]
> > assign_global_enable_per_cpu:ctrl=inherit,assign;mon=inherit,assign;group=uninitialized
> >
> >
> > > To (b), since the default resource group is used as default when the group
> > > is not provided it may be better to use "//" (or just "/", depending on system
> > > supporting just one of allocation or monitoring) instead of "uninitialized".
> >
> > To me, showing the default group would be confusing. It could give
> > the impression that only the default group is associated with the
> > [inherit] mode, even when multiple resource groups exist.
>
> I do not see how default group is associated with the "inherit" mode since inherit mode
> does not have a "group" property?

Yes. Got it.

Thanks,
Babu