Re: [PATCH v7 12/24] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow

["Moger, Babu" <babu.moger@amd.com>]

On 10/25/23 13:03, 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 housekeeping CPU is
> available.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Tested-by: Peter Newman <peternewman@google.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Signed-off-by: James Morse <james.morse@arm.com>

Reviewed-by: Babu Moger <babu.moger@amd.com>

> ---
> Changes since v3:
>  * typos fixed
> 
> Changes since v4:
>  * Made temporary variables unsigned
> 
> Changes since v5:
>  * Restructured cpumask_any_housekeeping() to avoid later churn.
> 
> Changes since v6:
>  * Update mbm_work_cpu/cqm_work_cpu when rescheduling.
> ---
>  arch/x86/kernel/cpu/resctrl/internal.h | 24 ++++++++++++++++++++++++
>  arch/x86/kernel/cpu/resctrl/monitor.c  | 20 +++++++++++++-------
>  2 files changed, 37 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 521afa016b05..33e24fcc8dd0 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>
>  
> @@ -56,6 +57,29 @@
>  /* Max event bits supported */
>  #define MAX_EVT_CONFIG_BITS		GENMASK(6, 0)
>  
> +/**
> + * cpumask_any_housekeeping() - Choose 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 housekeeping CPUs that don't use
> + * nohz_full, these are preferred.
> + */
> +static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
> +{
> +	unsigned int cpu, hk_cpu;
> +
> +	cpu = cpumask_any(mask);
> +	if (!tick_nohz_full_cpu(cpu))
> +		return cpu;
> +
> +	hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);
> +	if (hk_cpu < nr_cpu_ids)
> +		cpu = hk_cpu;
> +
> +	return cpu;
> +}
> +
>  struct rdt_fs_context {
>  	struct kernfs_fs_context	kfc;
>  	bool				enable_cdpl2;
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index cf512d4d383e..718770aea2af 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -783,7 +783,6 @@ static void mbm_update(struct rdt_resource *r, struct rdt_domain *d,
>  void cqm_handle_limbo(struct work_struct *work)
>  {
>  	unsigned long delay = msecs_to_jiffies(CQM_LIMBOCHECK_INTERVAL);
> -	int cpu = smp_processor_id();
>  	struct rdt_resource *r;
>  	struct rdt_domain *d;
>  
> @@ -794,8 +793,11 @@ void cqm_handle_limbo(struct work_struct *work)
>  
>  	__check_limbo(d, false);
>  
> -	if (has_busy_rmid(d))
> -		schedule_delayed_work_on(cpu, &d->cqm_limbo, delay);
> +	if (has_busy_rmid(d)) {
> +		d->cqm_work_cpu = cpumask_any_housekeeping(&d->cpu_mask);
> +		schedule_delayed_work_on(d->cqm_work_cpu, &d->cqm_limbo,
> +					 delay);
> +	}
>  
>  	mutex_unlock(&rdtgroup_mutex);
>  }
> @@ -805,7 +807,7 @@ void cqm_setup_limbo_handler(struct rdt_domain *dom, unsigned long delay_ms)
>  	unsigned long delay = msecs_to_jiffies(delay_ms);
>  	int cpu;
>  
> -	cpu = cpumask_any(&dom->cpu_mask);
> +	cpu = cpumask_any_housekeeping(&dom->cpu_mask);
>  	dom->cqm_work_cpu = cpu;
>  
>  	schedule_delayed_work_on(cpu, &dom->cqm_limbo, delay);
> @@ -815,7 +817,6 @@ void mbm_handle_overflow(struct work_struct *work)
>  {
>  	unsigned long delay = msecs_to_jiffies(MBM_OVERFLOW_INTERVAL);
>  	struct rdtgroup *prgrp, *crgrp;
> -	int cpu = smp_processor_id();
>  	struct list_head *head;
>  	struct rdt_resource *r;
>  	struct rdt_domain *d;
> @@ -839,7 +840,12 @@ void mbm_handle_overflow(struct work_struct *work)
>  			update_mba_bw(prgrp, d);
>  	}
>  
> -	schedule_delayed_work_on(cpu, &d->mbm_over, delay);
> +	/*
> +	 * Re-check for housekeeping CPUs. This allows the overflow handler to
> +	 * move off a nohz_full CPU quickly.
> +	 */
> +	d->mbm_work_cpu = cpumask_any_housekeeping(&d->cpu_mask);
> +	schedule_delayed_work_on(d->mbm_work_cpu, &d->mbm_over, delay);
>  
>  out_unlock:
>  	mutex_unlock(&rdtgroup_mutex);
> @@ -852,7 +858,7 @@ void mbm_setup_overflow_handler(struct rdt_domain *dom, unsigned long delay_ms)
>  
>  	if (!static_branch_likely(&rdt_mon_enable_key))
>  		return;
> -	cpu = cpumask_any(&dom->cpu_mask);
> +	cpu = cpumask_any_housekeeping(&dom->cpu_mask);
>  	dom->mbm_work_cpu = cpu;
>  	schedule_delayed_work_on(cpu, &dom->mbm_over, delay);
>  }

-- 
Thanks
Babu Moger