Re: [PATCH 4/4] x86/resctrl: optimize cpumask_any_housekeeping()

Yury Norov
2025-04-24T03:01:17Z

On Wed, Apr 23, 2025 at 02:29:48PM -0700, Reinette Chatre wrote:
> Hi Yury,
>
> On 4/7/25 8:38 AM, Yury Norov wrote:
> > From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> >
> > With the lack of cpumask_andnot_any_but(), users have to abuse
> > cpumask_nth() functions which are O(N*log(N)), comparing to O(N)
> > for cpumask_any().
> >
> > This series adds missing cpumask_andnot_any_but() and makes
> > cpumask_any_but() understanding the RESCTRL_PICK_ANY_CPU hint.
> > This simplifies cpumask_any_housekeeping() significantly.
>
> The "This series ..." language is more appropriate for the cover
> letter.
>
> This changelog could be something like:
>
> With the lack of cpumask_andnot_any_but(), cpumask_any_housekeeping()
> abused cpumask_nth() functions which are O(N*log(N)), compared to O(N)
> for cpumask_any().
>
> Update cpumask_any_housekeeping() to use the new cpumask_any_but() and
> cpumask_andnot_any_but(). These two functions understand RESCTRL_PICK_ANY_CPU
> and simplifies cpumask_any_housekeeping() significantly.
>
>
> Also, could you please have the subject of this patch start with an
> upper case: "x86/resctrl: Optimize cpumask_any_housekeeping()"?

Yep, I'll reword.

> > Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> > ---
> > arch/x86/kernel/cpu/resctrl/internal.h | 28 +++++++-------------------
> > 1 file changed, 7 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> > index 20c898f09b7e..1db02bab9743 100644
> > --- a/arch/x86/kernel/cpu/resctrl/internal.h
> > +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> > @@ -71,30 +71,16 @@
> > static inline unsigned int
> > cpumask_any_housekeeping(const struct cpumask *mask, int exclude_cpu)
> > {
> > - unsigned int cpu, hk_cpu;
> > -
> > - if (exclude_cpu == RESCTRL_PICK_ANY_CPU)
> > - cpu = cpumask_any(mask);
> > - else
> > - cpu = cpumask_any_but(mask, exclude_cpu);
> > -
> > - /* Only continue if tick_nohz_full_mask has been initialized. */
> > - if (!tick_nohz_full_enabled())
> > - return cpu;
> > -
> > - /* If the CPU picked isn't marked nohz_full nothing more needs doing. */
> > - if (cpu < nr_cpu_ids && !tick_nohz_full_cpu(cpu))
> > - return cpu;
> > + unsigned int cpu;
> >
> > /* Try to find a CPU that isn't nohz_full to use in preference */
> > - hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);
> > - if (hk_cpu == exclude_cpu)
> > - hk_cpu = cpumask_nth_andnot(1, mask, tick_nohz_full_mask);
> > -
> > - if (hk_cpu < nr_cpu_ids)
> > - cpu = hk_cpu;
> > + if (tick_nohz_full_enabled()) {
> > + cpu = cpumask_andnot_any_but(mask, tick_nohz_full_mask, exclude_cpu);
> > + if (cpu < nr_cpu_ids)
> > + return cpu;
> > + }
> >
> > - return cpu;
> > + return cpumask_any_but(mask, exclude_cpu);
> > }
> >
> > struct rdt_fs_context {
>
> This looks good to me. Thank you very much for doing this.

Thanks for the feedback! I'll send v2 before the end of week.

Thanks,
Yury