Re: [PATCH v3 08/19] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow

Ilpo Järvinen
2023-03-21T15:15:08Z

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.