Re: [PATCH v9 09/24] x86/resctrl: Use __set_bit()/__clear_bit() instead of open coding

[David Hildenbrand]

On 20.02.24 17:27, James Morse wrote:
> Hi David,
>
> On 20/02/2024 16:00, David Hildenbrand wrote:
> > On 13.02.24 19:44, James Morse wrote:
> > > The resctrl CLOSID allocator uses a single 32bit word to track which
> > > CLOSID are free. The setting and clearing of bits is open coded.
> > >
> > > Convert the existing open coded bit manipulations of closid_free_map
> > > to use __set_bit() and friends. These don't need to be atomic as this
> > > list is protected by the mutex.
>
> > > diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > > b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > > index dcffd1c4a476..bc6e0f83c847 100644
> > > --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > > +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > > @@ -111,7 +111,7 @@ void rdt_staged_configs_clear(void)
> > >     * - Our choices on how to configure each resource become progressively more
> > >     *   limited as the number of resources grows.
> > >     */
> >
> > That comment talks about "free CLOSIDs in a single integer". Once could think about
> > rephrasing that to "free CLOSIDs in a simple bitmap."
> >
> > > -static int closid_free_map;
> > > +static unsigned long closid_free_map;
> > >    static int closid_free_map_len;
> > >      int closids_supported(void)
> > > @@ -130,8 +130,8 @@ static void closid_init(void)
> > >          closid_free_map = BIT_MASK(rdt_min_closid) - 1;
> >
> > Now that we use "unsigned long", I was wondering for a second if we should use "proper"
> > bitmap functions here like
> >
> >      bitmap_fill(closid_free_map, rdt_min_closid);
> >
> > and converting the alloc path (replace ffs() in closid_alloc()):
> >
> >      closid = find_first_bit(closid_free_map, closid_free_map_len);
> >      if (closid == closid_free_map_len)
> >          return -ENOSPC;
> >      __clear_bit(closid, &closid_free_map);
> >
> > (would get rid of the closid-- in closid_alloc())
>
> Yup. I have this as something to post after all the MPAM changes as it's not necessary to
> get MPAM going. The patch[0] uses the bitmap APIs you suggest to remove the fixed limit on
> the number of CLOSID/PARTID.
> MPAM systems are being built with more than 32, but will work without that patch.

Make sense, thanks!

--
Cheers,

David / dhildenb