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    SubjectRe: [PATCH v3 08/19] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow
    On Mon, 20 Mar 2023, James Morse wrote:

    > The limbo and overflow code picks a CPU to use from the domain's list
    > of online CPUs. Work is then scheduled on these CPUs to maintain
    > the limbo list and any counters that may overflow.
    >
    > cpumask_any() may pick a CPU that is marked nohz_full, which will
    > either penalise the work that CPU was dedicated to, or delay the
    > processing of limbo list or counters that may overflow. Perhaps
    > indefinitely. Delaying the overflow handling will skew the bandwidth
    > values calculated by mba_sc, which expects to be called once a second.
    >
    > Add cpumask_any_housekeeping() as a replacement for cpumask_any()
    > that prefers housekeeping CPUs. This helper will still return
    > a nohz_full CPU if that is the only option. The CPU to use is
    > re-evaluated each time the limbo/overflow work runs. This ensures
    > the work will move off a nohz_full CPU once a houskeeping CPU is

    housekeeping

    > available.
    >
    > Signed-off-by: James Morse <james.morse@arm.com>
    > ---
    > arch/x86/kernel/cpu/resctrl/internal.h | 23 +++++++++++++++++++++++
    > arch/x86/kernel/cpu/resctrl/monitor.c | 17 ++++++++++++-----
    > include/linux/tick.h | 3 ++-
    > 3 files changed, 37 insertions(+), 6 deletions(-)
    >
    > diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
    > index 87545e4beb70..0b5fd5a0cda2 100644
    > --- a/arch/x86/kernel/cpu/resctrl/internal.h
    > +++ b/arch/x86/kernel/cpu/resctrl/internal.h
    > @@ -7,6 +7,7 @@
    > #include <linux/kernfs.h>
    > #include <linux/fs_context.h>
    > #include <linux/jump_label.h>
    > +#include <linux/tick.h>
    > #include <asm/resctrl.h>
    >
    > #define L3_QOS_CDP_ENABLE 0x01ULL
    > @@ -55,6 +56,28 @@
    > /* Max event bits supported */
    > #define MAX_EVT_CONFIG_BITS GENMASK(6, 0)
    >
    > +/**
    > + * cpumask_any_housekeeping() - Chose any cpu in @mask, preferring those that
    > + * aren't marked nohz_full
    > + * @mask: The mask to pick a CPU from.
    > + *
    > + * Returns a CPU in @mask. If there are houskeeping CPUs that don't use
    > + * nohz_full, these are preferred.
    > + */
    > +static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
    > +{
    > + int cpu, hk_cpu;
    > +
    > + cpu = cpumask_any(mask);
    > + if (tick_nohz_full_cpu(cpu)) {
    > + hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);

    Why cpumask_nth_and() is not enough here? ..._andnot() seems to alter
    tick_nohz_full_mask which doesn't seem desirable?


    --
    i.

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