Re: [RFC] mpam,x86,fs/resctrl: Generic schema description Proof of Concept

[Ben Horgan]

Hi Reinette,

On 5/29/26 19:06, Reinette Chatre wrote:
> Hi Everybody,
> 
> It has been a while since we discussed the resctrl changes required to support
> hardware that has controls with fine granularity or hardware that has multiple
> controls per resource. For reference, the most recent email discussion can
> be found at [1] with a summary of discussions in last year's plumbers slides [2].
> 
> I created a PoC that I believe supports what folks have agreed to so far. I
> hope this can help us to restart the discussion with the goal that resctrl gains
> support for upcoming hardware that require these features.

Thank you very much for doing this work. I believe this will be very useful for
MPAM and other architectures.

> 
> Request regarding this PoC
> ==========================
> 
> Please consider this PoC as a "direction check" on the schema description and multiple
> control discussions held thus far.
> 
> Could folks working on enabling new hardware requiring this capability please consider
> if this is something you can build on and how it should be improved to support these
> upcoming capabilities?
> 
> Opens
> =====
> 
> While the PoC aims to support what folks agreed on some opens remain:
> - I attempted to make some MPAM supporting changes but these are all just compile
>   tested. While MPAM should benefit from the new control properties I did not
>   initialize them on MPAM and did not attempt refactor to separate out
>   the architecture specific control properties (more on what this means later).
>   I did attempt some MPAM refactoring that duplicates the MPAM domain to the
>   control domain and monitoring domain lists in support of there being multiple
>   controls each with its own list of control domains but it is definitely not good
>   design.

I appreciate you including MPAM in this PoC. With this one line change I was
able to boot an MPAM system and mount resctrl which appears to behave correctly.
We can consider how best to do the code design later.

--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -1697,6 +1697,8 @@ int mpam_resctrl_setup(void)

        /* Initialise the resctrl structures from the classes */
        for_each_mpam_resctrl_control(res, rid) {
+               INIT_LIST_HEAD(&res->resctrl_res.controls); // list_empty needs
to work
+
                if (!res->class)
                        continue;       // dummy resource

I plumbed in support for the MB_MIN resource schema which also works under light
testing. The only fs resctrl code change I needed was:

--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -483,6 +483,9 @@ static inline u32 resctrl_get_default_ctrlval(struct
resctrl_ctrl *ctrl)
        case RESCTRL_CTRL_BITMAP:
                return BIT_MASK(ctrl->cache.cbm_len) - 1;
        case RESCTRL_CTRL_SCALAR:
+               if (ctrl->name == RESCTRL_CTRL_NAME_MIN)
+                       return ctrl->membw.min_bw;
+
                return ctrl->membw.max_bw;
        }


At least on MPAM systems, we use a default of 0 for minimum bandwidth controls
as the maximum bandwidth controls only take effect if their value is higher than
the minimum bandwidth value. I have specialised this on the ctrl->name which
breaks your ctrl->type based classification but that's fixable by just adding a
default field to membw.


> - No support for emulated controls (yet). The PoC is quite large already
>   but I think it can be used as a base for emulated controls for which the software
>   controller could be a potential first customer. In this PoC mounting with
>   software controller will still display the original controller's properties.
> - One open that needs to be addressed as part of support for emulated controls is
>   how best to display emulation relationship via resctrl hierarchy.

What does emulated controls mean here? Is there some previous discussion you
could point me at?

> - No support for "read-modify-write" usage of schemata file. This is where we
>   discussed (without agreement) on possibly introducing the "#" prefix to schemata
>   file entries. This PoC does not support this prefix and the current assumption/expectation
>   is that when user space changes a configuration only the new control values are
>   written to schemata file. I thus do not have a plan to support this so please
>   share opinions in this regard if you have some.

There is now less motivation from the MPAM side for this than when this was
initially discussed. In pre-upstream versions of the MPAM patches a change in
the MB resource control value would change both the mpam h/w mbw_min and mbw_max
values but now (on non-broken h/w) we just change the mbw_max. (mbw_min kept at 0).

However, it would be useful not to be limited by percentages. In my quick
experimentation with your patches I used a percentage value for MB_MIN but it
would be best to move away from this. For new controls I think we can mandate
that user space has to discover the resolution from the info directly but how
can we retrofit this. For MPAM, MB and MB_MAX, would control the same things.
Could we just add MB_MAX with a h/w friendly scale and then reflect changes in
MB_MAX in MB and vica versa with MB taking precedent if both are set? Old
software can continue setting MB can move to using MB_MAX and take advantage of
the improved control. (I don't think we should expose the MPAM hardware value
directly as it has confusion over whether all 1s is 100% or not and we'd like to
have something generic and friendly to the user.)

> - Controls are independent for now. This means that, for example, if a resource
>   supports a "MIN" and "MAX" control then this implementation would allow user to
>   set the "maximum" control values to be less than the "minimum" control values.

I think this is ok as long as adding support for new controls in resctrl doesn't
change the existing behaviour. In MPAM we dodged this by introducing MB as only
affecting the h/w mbw_max and not mbw_min (as mentioned above).

> - PoC supports the "bitmap" control but does not (yet) expose properties of a bitmap
>   control to the new info/<resource>/resource_schemata directory.
> 
> Accessing PoC
> =============
> 
> Please consider the PoC as a rough draft. It has only been compile tested for Arm
> and known to be incomplete in Arm support. To help with experimenting I only
> fully adapted the Intel MBA resource to demo two dummy additional MBA controls.
> All architectures should immediately benefit from the new schema descriptions
> and new info/MB/resource_schemata hierarchy.
> 
> I considered the patches self too many for email. Instead, the PoC can be found at:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/reinette/linux.git branch resctrl/controls_rfc_v1
> 
> The work is based on v7.1-rc2 that also includes the following series (two of which has
> since been queued) included:
> 
> "selftests/resctrl: Fixes and improvements focused on Intel platforms"
> https://lore.kernel.org/lkml/cover.1775266384.git.reinette.chatre@xxxxxxxxx/
> 
> "x86,fs/resctrl: Improve resctrl quality and consistency"
> https://lore.kernel.org/lkml/cover.1777419024.git.reinette.chatre@xxxxxxxxx/
> 
> "x86,fs/resctrl: Pave the way for MPAM counter assignment"
> https://lore.kernel.org/lkml/20260506082855.3694761-1-ben.horgan@xxxxxxx/
> 
> 
> Primary resctrl fs data structure changes
> =========================================
> 
> Introduces a control represented by struct resctrl_ctrl that looks as below. To make
> the changes easier to follow I kept some of the original names to help communicate
> where familiar data structures land.
> 
> What to notice about a control is that it has some common properties required
> from all controls (scope, type, etc.) and then depending on the type of control
> (RESCTRL_CTRL_BITMAP or RESCTRL_CTRL_SCALAR) there are type specific properties.
> 
> /**
>  * struct resctrl_ctrl - A resource control
>  * @entry:	List entry of rdt_resource::controls
>  * @scope:	Scope of the resource that this control allocates
>  * @domains:	RCU list of all control domains
>  * @type:	The control type that determines the properties of the control,
>  *		format string for displaying control values to user space, and
>  *		parser of control values provided by user space.
>  * @name:	Name of the control. Appended to final resource name
>  *		(rdt_resource_final::name) to create final schema entry.
>  *		Specifically, "rdt_resource_final::name"_"resctrl_ctrl::name".
>  *		For example, with resource name "MB" and control name "MAX" the
>  *		schema entry will be "MB_MAX".
>  * @cache:	Cache allocation control properties.
>  * @membw:	Bandwidth control properties.
>  */
> struct resctrl_ctrl {
> 	struct list_head	entry;
> 	enum resctrl_scope	scope;
> 	struct list_head	domains;
> 	enum resctrl_ctrl_type	type;
> 	enum resctrl_ctrl_name	name;
> 	union {
> 		struct resctrl_cache	cache;
> 		struct resctrl_membw	membw;
> 	};
> };
> 
> Two members summarize how this new structure fits into the rest of resctrl:
> a) resctrl_ctrl::entry 
>    Since a resource can support multiple controls there is a new list
>    in struct rdt_resource named "controls" that contains the list of all
>    controls supported by the resource.
> b) resctrl_ctrl::domains
>    Instead of the list of control domains belonging to a resource they
>    now belong to the control self. By doing so resctrl can support resource
>    controls at different scope for the same resource. This is intended to
>    support some upcoming MPAM and RISC-V usages.

Please can you expand a bit on  part b).

In an MPAM system we consider 3 resctrl resources, RDT_RESOURCE_L3,
RDT_RESOURCE_L2 and RDT_RESOURCE_MBA which correspond to the L3 caches, L2
caches and memory bandwidth on egress from the L3 caches. The domain for each of
these corresponds to the instance of the resource. That is, for RDT_RESOURCE_L2
there is a resource for each L2 instance, similarly for L3, and for
RDT_RESOURCE_MBA there is a domain for each L3 cache. If we were to add suport
for controls on a new cache level, say the L4, then I'd expect to add a new
resource. For memory bandwidth, we'd like to be able to control b/w on the L2
egress (e.g. in a DSU). Wouldn't this too be a separate resource or would this
be a new set of controls on the same resource?

New controls on the same resource
MB_MIN2
MB_MAX2
MB_PROP2
...

or
MB2_MIN
MB2_MAX
MB2_PROP

AFAIK, the DSU h/w just supports proportional bandwidth controls at the moment
but we should consider what to do about the potential naming.

In the MPAM driver, we collect MSC into components (based on instances) and
those into classes (components of the same type). Currently, a resource is
mapped to a single class. (Two resources may map to the same class.)

I expect it is useful in the memory region and sub numa cases but I'd still
expect the common case to be that the domains are the same within a control. Or
am I missing something?

> 	
> Example architectural data structure changes
> ============================================
> 
> An architecture can use the new control by following a similar pattern to
> resource and domain use by architectures. Consider the following for x86
> where a new architecture specific struct resctrl_hw_ctrl includes 
> struct resctrl_ctrl and any architecture private data needed to support
> the control:
> 
> /*
>  * struct resctrl_hw_ctrl - Arch private properties of a resource control
>  * @r_ctrl:	Control properties exposed to resctrl file system
>  * @msr_base:	Base MSR address where control values should be programmed
>  * @msr_update:	Function pointer to update control values
>  */
> struct resctrl_hw_ctrl {
> 	struct resctrl_ctrl	r_ctrl;
> 	unsigned int		msr_base;
> 	void			(*msr_update)(struct msr_param *m);
> };
> 
> Structure of patch series
> =========================
> 
> As a PoC the series is not perfectly structured but to help navigate this work
> on a high level the changes can be categorized as follows:
> 
> Patch 1 to 11:
> With a vision of what a "control" is, remove unused/unnecessary
> members, make clear what is a *resource* property vs a *control*
> property, do some renaming to help with the PoC.

A few of the changes are generic cleanup and could hopefully be dealt with
before decisions on the larger PoC are made. I see:
fs/resctrl: Remove unused resctrl_membw::mb_map
x86,fs/resctrl: Remove "arch_needs_linear"
Perhaps a few more.
> 
> Patch 12:
> Introduce struct resctrl_ctrl and re-arrange existing struct rdt_resource
> members to form part of new rdt_resource::ctrl
> 
> Patch 13 to 44:
> A lot of wrangling to introduce struct resctrl_ctrl to all code that needs
> to work with a control and/or domain without assuming that the control is
> the one and only control embedded in the resource it belongs to. Essentially,
> a lot of changes passing the control around in addition to the resource/domain.

You mention a few times in the commit message that you expect the cache
resources to only have one control. On MPAM we have CMAX (and there looks to be
a RISC-V equivalent) where the total number of bytes in the cache for a given
closid is limited. The allocation must still respect the CPBM bitmap though.
Looking at the code though I don't see much problem in adding this as an
additional control. The assumption that these patches is making is not that
there is only one control for cache resources but rather that cache portions are
managed by the default cache resource control. Am I missing something or does
that assessment make sense to you?

I have been looking at adding CMAX control to resctrl and will have a go at
basing what I have so far on top of this series.

> 
> Patch 45:
> Switch the single struct resctrl_ctrl member of struct rdt_resource to be
> a list of struct resctrl_ctrl.
> 
> Patch 47 to 49:
> Introduce new info/<resource>/resource_schemata hierarchy to first only
> consist of properties already known to resctrl fs.
> 
> Patch 50 to 52:
> Introduce the new control properties per [1], initialize them for x86,
> and expose them via info/<resource>/resource_schemata
> 
> Patch 53:
> Let the new struct resctrl_hw_ctrl contain architecture's control properties.
> 
> Patch 54:
> Teach resctrl fs about "MIN" and "MAX" controls.
> 
> Patch 55:
> Sample of "MIN" and "MAX" memory bandwidth controls for x86.
> > Example interactions
> ====================
> 
> This series can be used on an x86 system where it will show two new dummy controls
> where it is possible to interact with the new controls.
> For example:
> 
> # cat schemata 
>         MB_MAX:0=100;1=100
>         MB_MIN:0=100;1=100
>         MB:0=100;1=100
>         L3:0=fff;1=fff
> # echo 'MB_MIN:0=50' > schemata 
> # cat schemata 
>         MB_MAX:0=100;1=100
>         MB_MIN:0=50;1=100
>         MB:0=100;1=100
>         L3:0=fff;1=fff
> 
> Writing to the dummy control will call a dummy callback that just prints to the
> kernel log:
> "resctrl: Updata temporary MIN control on domain 0 with user value 50"
> 
> 
> Example output of info/MB/:
> /sys/fs/resctrl/info/MB/thread_throttle_mode:max
> /sys/fs/resctrl/info/MB/num_closids:15
> /sys/fs/resctrl/info/MB/delay_linear:1
> /sys/fs/resctrl/info/MB/min_bandwidth:10
> /sys/fs/resctrl/info/MB/resource_schemata/MB/resolution:100
> /sys/fs/resctrl/info/MB/resource_schemata/MB/tolerance:5
> /sys/fs/resctrl/info/MB/resource_schemata/MB/type:scalar
> /sys/fs/resctrl/info/MB/resource_schemata/MB/min:10
> /sys/fs/resctrl/info/MB/resource_schemata/MB/scale:1
> /sys/fs/resctrl/info/MB/resource_schemata/MB/scope:L3
> /sys/fs/resctrl/info/MB/resource_schemata/MB/unit:all
> /sys/fs/resctrl/info/MB/resource_schemata/MB/max:100
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/resolution:100
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/tolerance:5
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/type:scalar
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/min:10
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/scale:1
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/scope:L3
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/unit:all
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MIN/max:100
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/resolution:100
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/tolerance:5
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/type:scalar
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/min:10
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/scale:1
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/scope:L3
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/unit:all
> /sys/fs/resctrl/info/MB/resource_schemata/MB_MAX/max:100
> /sys/fs/resctrl/info/MB/bandwidth_gran:10
> > Any feedback is appreciated.

Overall, this looks to be a big step in the right direction.

Thanks,

Ben

> 
> Reinette
> 
> [1] https://lore.kernel.org/lkml/aPtfMFfLV1l%2FRB0L@xxxxxxxxxxxxxxx/
> [2] https://lpc.events/event/19/contributions/2093/attachments/1958/4172/resctrl%20Microconference%20LPC%202025%20Tokyo.pdf