Re: [PATCH v3 17/19] x86/resctrl: Allow overflow/limbo handlers to be scheduled on any-but cpu

[Ilpo Järvinen]

On Mon, 20 Mar 2023, James Morse wrote:

> When a CPU is taken offline resctrl may need to move the overflow or
> limbo handlers to run on a different CPU.
> 
> Once the offline callbacks have been split, cqm_setup_limbo_handler()
> will be called while the CPU that is going offline is still present
> in the cpu_mask.
> 
> Pass the CPU to exclude to cqm_setup_limbo_handler() and
> mbm_setup_overflow_handler(). These functions can use a variant of
> cpumask_any_but() when selecting the CPU. -1 is used to indicate no CPUs
> need excluding.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> ---
> Changes since v2:
>  * Rephrased a comment to avoid a two letter bad-word. (we)
>  * Avoid assigning mbm_work_cpu if the domain is going to be free()d
>  * Added cpumask_any_housekeeping_but(), I dislike the name
> ---
>  arch/x86/kernel/cpu/resctrl/core.c     |  8 +++--
>  arch/x86/kernel/cpu/resctrl/internal.h | 37 ++++++++++++++++++++--
>  arch/x86/kernel/cpu/resctrl/monitor.c  | 43 +++++++++++++++++++++-----
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c |  6 ++--
>  include/linux/resctrl.h                |  3 ++
>  5 files changed, 83 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 8e25ea49372e..aafe4b74587c 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -582,12 +582,16 @@ static void domain_remove_cpu(int cpu, struct rdt_resource *r)
>  	if (r == &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl) {
>  		if (is_mbm_enabled() && cpu == d->mbm_work_cpu) {
>  			cancel_delayed_work(&d->mbm_over);
> -			mbm_setup_overflow_handler(d, 0);
> +			/*
> +			 * exclude_cpu=-1 as this CPU has already been removed
> +			 * by cpumask_clear_cpu()d
> +			 */
> +			mbm_setup_overflow_handler(d, 0, RESCTRL_PICK_ANY_CPU);
>  		}
>  		if (is_llc_occupancy_enabled() && cpu == d->cqm_work_cpu &&
>  		    has_busy_rmid(r, d)) {
>  			cancel_delayed_work(&d->cqm_limbo);
> -			cqm_setup_limbo_handler(d, 0);
> +			cqm_setup_limbo_handler(d, 0, RESCTRL_PICK_ANY_CPU);
>  		}
>  	}
>  }
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 3eb5b307b809..47838ba6876e 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -78,6 +78,37 @@ static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
>  	return cpu;
>  }
>  
> +/**
> + * cpumask_any_housekeeping_but() - Chose any cpu in @mask, preferring those
> + *			            that aren't marked nohz_full, excluding
> + *				    the provided CPU
> + * @mask:	The mask to pick a CPU from.
> + * @exclude_cpu:The CPU to avoid picking.
> + *
> + * Returns a CPU from @mask, but not @but. If there are houskeeping CPUs that
> + * don't use nohz_full, these are preferred.
> + * Returns >= nr_cpu_ids if no CPUs are available.
> + */
> +static inline unsigned int
> +cpumask_any_housekeeping_but(const struct cpumask *mask, int exclude_cpu)
> +{
> +	int cpu, hk_cpu;
> +
> +	cpu = cpumask_any_but(mask, exclude_cpu);
> +	if (tick_nohz_full_cpu(cpu)) {
> +		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);

I'm left to wonder if it's okay to alter tick_nohz_full_mask in resctrl 
code??


-- 
 i.