Re: [PATCH v7 21/24] x86/resctrl: Allow overflow/limbo handlers to be scheduled on any-but cpu

[Reinette Chatre]

Hi James,

On 10/25/2023 11:03 AM, 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.
> 
> A subsequent patch moves these calls to be before CPUs have been removed,
> so this exclude_cpus behaviour is temporary.

Note "A subsequent patch". Please do go over your entire series. I may not
have noticed all.

> 
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Tested-by: Peter Newman <peternewman@xxxxxxxxxx>
> Reviewed-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
> 
> Changes since v3:
>  * Marked an explanatory comment as temporary as the subsequent patch is
>    no longer adjacent.
> 
> Changes since v4:
>  * Check against RESCTRL_PICK_ANY_CPU instead of -1.
>  * Leave cqm_work_cpu as nr_cpu_ids when no CPU is available.
>  * Made cpumask_any_housekeeping_but() more readable.
> 
> Changes since v5:
>  * Changes in captialisation, and a typo.
>  * Merged cpumask helpers.
> 
> Changes since v6:
>  * Added the missing dom parameter to some kernel doc.
>  * Re-added use of cpumask_any_but(),
>  * Expanded comment above cpumask_any_housekeeping(),
>  * Added some more comments for good measure.
>  * Added explicit IS_ENABLED() check as gcc-12 doesn't seem to work this out.
> ---
>  arch/x86/kernel/cpu/resctrl/core.c        |  8 +++--
>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c |  2 +-
>  arch/x86/kernel/cpu/resctrl/internal.h    | 33 ++++++++++++++----
>  arch/x86/kernel/cpu/resctrl/monitor.c     | 42 ++++++++++++++++++-----
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c    |  6 ++--
>  include/linux/resctrl.h                   |  2 ++
>  6 files changed, 72 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 1a74e9c47416..7e44f2c40897 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -586,12 +586,16 @@ 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);
> +			/*
> +			 * temporary: 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(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/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> index a033e8e32108..64db51455df3 100644
> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> @@ -552,7 +552,7 @@ void mon_event_read(struct rmid_read *rr, struct rdt_resource *r,
>  		return;
>  	}
>  
> -	cpu = cpumask_any_housekeeping(&d->cpu_mask);
> +	cpu = cpumask_any_housekeeping(&d->cpu_mask, RESCTRL_PICK_ANY_CPU);
>  
>  	/*
>  	 * cpumask_any_housekeeping() prefers housekeeping CPUs, but
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index c4c1e1909058..f5fff2f0d866 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -61,19 +61,36 @@
>   * cpumask_any_housekeeping() - Choose any CPU in @mask, preferring those that
>   *			        aren't marked nohz_full
>   * @mask:	The mask to pick a CPU from.
> + * @exclude_cpu:The CPU to avoid picking.
>   *
> - * Returns a CPU in @mask. If there are housekeeping CPUs that don't use
> - * nohz_full, these are preferred.
> + * Returns a CPU from @mask, but not @exclude_cpu. If there are housekeeping
> + * CPUs that don't use nohz_full, these are preferred. Pass
> + * RESCTRL_PICK_ANY_CPU to avoid excluding any CPUs.
> + *
> + * When a CPU is excluded, returns >= nr_cpu_ids if no CPUs are available.
>   */
> -static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
> +static inline unsigned int
> +cpumask_any_housekeeping(const struct cpumask *mask, int exclude_cpu)
>  {
>  	unsigned int cpu, hk_cpu;
>  
> -	cpu = cpumask_any(mask);
> -	if (!tick_nohz_full_cpu(cpu))
> +	if (exclude_cpu == RESCTRL_PICK_ANY_CPU)
> +		cpu = cpumask_any(mask);
> +	else
> +		cpu = cpumask_any_but(mask, exclude_cpu);
> +
> +	if (!IS_ENABLED(CONFIG_NO_HZ_FULL))
> +		return cpu;

It is not clear to me how cpumask_any_but() failure is handled.

cpumask_any_but() returns ">= nr_cpu_ids if no cpus set" ...

> +
> +	/* If the CPU picked isn't marked nohz_full, we're done */

Please don't impersonate code.

> +	if (cpu <= nr_cpu_ids && !tick_nohz_full_cpu(cpu))
>  		return cpu;

Is this intended to be "cpu < nr_cpu_ids"? But that would have
code continue ... so maybe it needs explicit error check of
cpumask_any_but() failure with an earlier exit?

>  
> +	/* 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;
>  

Reinette