Re: [PATCH v3 17/38] x86/resctrl: Stop using the for_each_*_rdt_resource() walkers

[Reinette Chatre]

Hi James,

On 7/1/24 11:16 AM, James Morse wrote:
> Hi Reinette,
>
> On 28/06/2024 17:48, Reinette Chatre wrote:
> > On 6/14/24 8:00 AM, James Morse wrote:
> > > The for_each_*_rdt_resource() helpers walk the architecture's array
> > > of structures, using the resctrl visible part as an iterator. These
> > > became over-complex when the structures were split into a
> > > filesystem and architecture-specific struct. This approach avoided
> > > the need to touch every call site.
> > >
> > > Once the filesystem parts of resctrl are moved to /fs/, both the
> > > architecture's resource array, and the definition of those structures
> > > is no longer accessible. To support resctrl, each architecture would
> > > have to provide equally complex macros.
> > >
> > > Change the resctrl code that uses these to walk through the resource_level
> > > enum and check the mon/alloc capable flags instead. Instances in core.c,
> > > and resctrl_arch_reset_resources() remain part of x86's architecture
> > > specific code.
>
> > > diff --git a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> > > b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> > > index aacf236dfe3b..ad20822bb64e 100644
> > > --- a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> > > +++ b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> > > @@ -854,7 +855,11 @@ bool rdtgroup_pseudo_locked_in_hierarchy(struct rdt_domain *d)
> > >         * First determine which cpus have pseudo-locked regions
> > >         * associated with them.
> > >         */
> > > -    for_each_alloc_capable_rdt_resource(r) {
> > > +    for (i = 0; i < RDT_NUM_RESOURCES; i++) {
> > > +        r = resctrl_arch_get_resource(i);
> > > +        if (!r->alloc_capable)
> > > +            continue;
> > > +
> >
> > This looks like enough duplicate boilerplate for a new macro. For simplicity the
> > macro could require two arguments with enum resctrl_res_level also provided?
>
> I was hoping to escape from these clever macros! If you think this is too much:
> - we'd need to come up with another name, as the arch code keeps the existing definition.
> - to avoid touching every caller, it needs doing without an explicit iterator variable.
>
> I guess the cleanest thing is to redefine the existing macros to use
> resctrl_arch_get_resource(). Putting this in include/linxu/resctrl.h at least avoids each
> architecture needing to define these, or forcing it to use an array.
>
> The result is slightly more readable than the current version:
> | #define for_each_rdt_resource(_r)                              \
> |        for (_r = resctrl_arch_get_resource(0);                 \
> |             _r->rid < RDT_NUM_RESOURCES;                       \
> |             _r = resctrl_arch_get_resource(_r->rid + 1))
>
> This leans heavily on resctrl_arch_get_resource() not being able to return NULL, and
> having to return a dummy resource that is neither alloc nor mon capable. We may need to
> revisit that if it becomes a burden for the arch code.

Replacing the repetitive four lines of code with a single line seems good to me.

resctrl_arch_get_resource() being able to return NULL is introduced in this series but
I am not seeing any handling of a possible NULL value. Not being able to return NULL thus
already seems a requirement?

Reinette