Re: [PATCH v1 02/31] x86/resctrl: Add a helper to avoid reaching into the arch code resource list

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Apr 16, 2024 at 03:44:29PM -0500, Moger, Babu wrote:
> Hi Dave,
> 
> On 4/16/24 11:15, Dave Martin wrote:
> > On Mon, Apr 15, 2024 at 03:28:18PM -0500, Moger, Babu wrote:
> >> Hi James/Dave,
> >>
> >> On 3/21/24 11:50, James Morse wrote:
> >>> Resctrl occasionally wants to know something about a specific resource,
> >>> in these cases it reaches into the arch code's rdt_resources_all[]
> >>> array.
> >>>
> >>> Once the filesystem parts of resctrl are moved to /fs/, this means it
> >>> will need visibility of the architecture specific struct
> >>> resctrl_hw_resource definition, and the array of all resources.
> >>> All architectures would also need a r_resctrl member in this struct.
> >>>
> >>> Instead, abstract this via a helper to allow architectures to do
> >>> different things here. Move the level enum to the resctrl header and
> >>> add a helper to retrieve the struct rdt_resource by 'rid'.
> >>>
> >>> resctrl_arch_get_resource() should not return NULL for any value in
> >>> the enum, it may instead return a dummy resource that is
> >>> !alloc_enabled && !mon_enabled.
> >>
> >> Nit.
> >> You may want to drop the second half of the statement. We don't have a
> >> dummy resource.
> > 
> > I guess not, but MPAM will, although I haven't fully understood the
> > logic.  See:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_resctrl.c?h=mpam/snapshot/v6.7-rc2
> > 
> > (Search for "dummy".)
> > 
> > 
> > In any case, the statement above is part of the definition of the new
> > interface: the resctrl core code is going to explicitly need to cope
> > with a dummy resource being returned, and the arch code is required
> > to return a pointer to something and not NULL.
> > 
> > So I would say that it is appropriate (or, at the very least, harmless)
> > to keep that statement here?
> 
> Ok. fine.
> > 
> >>
> >>>
> >>> Signed-off-by: James Morse <james.morse@arm.com>
> >>> ---
> >>>  arch/x86/kernel/cpu/resctrl/core.c        | 10 +++++++++-
> >>>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  2 +-
> >>>  arch/x86/kernel/cpu/resctrl/internal.h    | 10 ----------
> >>>  arch/x86/kernel/cpu/resctrl/monitor.c     |  8 ++++----
> >>>  arch/x86/kernel/cpu/resctrl/rdtgroup.c    | 15 +++++++--------
> >>>  include/linux/resctrl.h                   | 17 +++++++++++++++++
> >>>  6 files changed, 38 insertions(+), 24 deletions(-)
> >>>
> > 
> > [...]
> > 
> >>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> >>> index 1767c1affa60..45372b6a6215 100644
> >>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> >>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > 
> > [...]
> > 
> >>> @@ -2625,10 +2625,10 @@ static void schemata_list_destroy(void)
> >>>  
> >>>  static int rdt_get_tree(struct fs_context *fc)
> >>>  {
> >>> +	struct rdt_resource *l3 = resctrl_arch_get_resource(RDT_RESOURCE_L3);
> >>
> >> Its is probably best to keep the resource name as r here to be consistent
> >> with other changes.
> >>
> >>>  	struct rdt_fs_context *ctx = rdt_fc2context(fc);
> >>>  	unsigned long flags = RFTYPE_CTRL_BASE;
> >>>  	struct rdt_domain *dom;
> >>> -	struct rdt_resource *r;
> >>>  	int ret;
> >>>  
> >>>  	cpus_read_lock();
> >>> @@ -2701,8 +2701,7 @@ static int rdt_get_tree(struct fs_context *fc)
> >>>  		resctrl_mounted = true;
> >>>  
> >>>  	if (is_mbm_enabled()) {
> >>> -		r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> >>> -		list_for_each_entry(dom, &r->domains, list)
> >>> +		list_for_each_entry(dom, &l3->domains, list)
> >>>  			mbm_setup_overflow_handler(dom, MBM_OVERFLOW_INTERVAL,
> >>>  						   RESCTRL_PICK_ANY_CPU);
> >>>  	}
> >>> @@ -3878,7 +3877,7 @@ static int rdtgroup_show_options(struct seq_file *seq, struct kernfs_root *kf)
> > 
> > [...]
> > 
> >> Thanks
> >> Babu Moger
> > 
> > [...]
> > 
> > Yes, this does look a bit odd.
> > 
> > This looks like a no-op change to me -- I think that
> > resctrl_arch_get_resource() is supposed to be without side-effects,
> > so I would have expected this to be a one-line change at the assignment
> > to r, with no particular need for renaming r.
> > 
> > Does that make sense to you, or is there some complexity I'm not
> > noticing here?
> 
> No other complexity..  Just keep the variable name as r.
> 
> struct rdt_resource *r = resctrl_arch_get_resource(RDT_RESOURCE_L3);

Ack; I went for the minimum-diffstat option in the end, so kept the
declaration and initialiser separate.  If you have a strong view on that
though, please shout.

Cheers
---Dave