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

Dave Martin
2024-04-16T16:15:15Z

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?

>
> >
> > 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?

Cheers
---Dave