Re: [PATCH v4] fs/resctrl: Add CONFIG_RESCTRL_ASSIGN_FIXED Kconfig entry

[Ben Horgan <ben.horgan@arm.com>]

Hi Reinette,

On Mon, Mar 02, 2026 at 03:32:39PM -0800, Reinette Chatre wrote:
> Hi Ben,
> 
> On 3/2/26 2:02 AM, Ben Horgan wrote:
> > Hi Boris,
> > 
> > On 3/1/26 10:49, Borislav Petkov wrote:
> >> On Wed, Feb 04, 2026 at 04:19:52PM +0000, Ben Horgan wrote:
> >>> diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
> >>> index 572a9925bd6c..4e9f55ee3267 100644
> >>> --- a/fs/resctrl/monitor.c
> >>> +++ b/fs/resctrl/monitor.c
> >>> @@ -1451,6 +1451,12 @@ ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of, char *buf,
> >>>  	}
> >>>  
> >>>  	if (enable != resctrl_arch_mbm_cntr_assign_enabled(r)) {
> >>> +		if (IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED)) {
> >>
> >> Does it need to be a Kconfig entry or you can figure out programatically from
> >> the architecture that that is the case and avoid yet another CONFIG_ knob?
> >>
> >> From reading this, I'm thinking
> >>
> >> 		if (IS ARM64) {
> >>
> >> should do or...?
> > 
> > Yes, we don't necessarily need the Kconfig and we don't necessarily need this condition at all in
> > resctrl_mbm_assign_mode_write(), we do need the existing one in resctrl_mbm_assign_mode_show() though.
> 
> It is not obvious to me why the one in resctrl_mbm_assign_mode_show() is required if
> a new arch helper could suffice in other parts 

I think a new arch helper could cover both.

> 
> > 
> > The Kconfig was effectively added in:
> > 
> > commit 3b497c3f4f04 ("fs/resctrl: Introduce the interface to display monitoring modes")
> > 
> > and I was just filling in the missing piece. Most arch specific things in resctrl seem
> > to be dealt with using resctrl_arch_* hooks. Would it be preferable to create a new one of these?
> 
> It looks to me as though this patch fits in with the other series you posted at
> https://lore.kernel.org/lkml/20260225201905.3568624-1-ben.horgan@arm.com/
> 
> When looking at this patch together with the other series there may indeed be an arch
> helper that can replace CONFIG_RESCTRL_ASSIGN_FIXED but as I mentioned in that series
> the MPAM capabilities and strategies for resctrl to support them are not clear to me
> at this time.

If you like, I can drop this patch and tag a resctrl_arch_ hook based hook
version on to that series when I respin. Do you prefer a new hook,
resctrl_arch_mbm_cntr_assign_fixed(), or using/abusing the error return of
resctrl_arch_mbm_assign_set()?

> 
> 
> > It could be resctrl_arch_mbm_cntr_assign_fixed() although we seem to be getting too many
> > of these too. Perhaps we could extend the meaning of another hook, resctrl_arch_mbm_assign_set() could
> 
> Could you please elaborate which category of helper is getting to be too many? We can surely
> explore improvements.

Nothing major or anything that necessarily needs to change. Just that for each
new feature we get a few new resctrl_arch_* and they all need to be updated in
the MPAM driver even if the feature isn't used.

> 
> > return -EOPNOTSUPP to indicate to resctrl that the counter assignment mode is not configurable.
> > 
> 
> Reinette
> 

Thanks,

Ben