Re: [PATCH 4/4] sched_ext: Introduce NUMA aware idle cpu kfunc helpers

[Yury Norov <yury.norov@gmail.com>]

On Mon, Dec 09, 2024 at 11:40:58AM +0100, Andrea Righi wrote:
> Add the following kfunc's to provide scx schedulers direct access to
> per-node idle cpumasks information:
> 
>  const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
>  const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
>  s32 scx_bpf_pick_idle_cpu_node(int node,
>                                 const cpumask_t *cpus_allowed, u64 flags)
>  int scx_bpf_cpu_to_node(s32 cpu)
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/ext.c                       | 96 +++++++++++++++++++++++-
>  tools/sched_ext/include/scx/common.bpf.h |  4 +
>  tools/sched_ext/include/scx/compat.bpf.h | 19 +++++
>  3 files changed, 117 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index d0d57323bcfc..ea7cc481782c 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -433,6 +433,7 @@ struct sched_ext_ops {
>  	 * - scx_bpf_select_cpu_dfl()
>  	 * - scx_bpf_test_and_clear_cpu_idle()
>  	 * - scx_bpf_pick_idle_cpu()
> +	 * - scx_bpf_pick_idle_cpu_node()
>  	 *
>  	 * The user also must implement ops.select_cpu() as the default
>  	 * implementation relies on scx_bpf_select_cpu_dfl().
> @@ -955,6 +956,8 @@ static struct cpumask *get_idle_cpumask_node(int node)
>  	if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
>  		return idle_masks[0]->cpu;
>  
> +	if (node < 0 || node >= num_possible_nodes())
> +		return NULL;

1. This sanity should go before the check above.
2. In-kernel users don't need to do sanity checks. BPF users should,
   but for them you need to move it in BPF wrapper.
3. -1 is a valid parameter, means NUMA_NO_NODE. 

>  	return idle_masks[node]->cpu;
>  }
>  
> @@ -963,6 +966,8 @@ static struct cpumask *get_idle_smtmask_node(int node)
>  	if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
>  		return idle_masks[0]->smt;
>  
> +	if (node < 0 || node >= num_possible_nodes())
> +		return NULL;
>  	return idle_masks[node]->smt;
>  }
>  
> @@ -7469,6 +7474,16 @@ __bpf_kfunc u32 scx_bpf_nr_cpu_ids(void)
>  	return nr_cpu_ids;
>  }
>  
> +/**
> + * scx_bpf_cpu_to_node - Return the NUMA node the given @cpu belongs to
> + */
> +__bpf_kfunc int scx_bpf_cpu_to_node(s32 cpu)
> +{
> +	if (cpu < 0 || cpu >= nr_cpu_ids)
> +		return -EINVAL;
> +	return cpu_to_node(cpu);
> +}

I believe this wrapper should be declared somewhere in
kernel/sched/topology.c, and better be a separate patch.

> +
>  /**
>   * scx_bpf_get_possible_cpumask - Get a referenced kptr to cpu_possible_mask
>   */
> @@ -7499,11 +7514,32 @@ __bpf_kfunc void scx_bpf_put_cpumask(const struct cpumask *cpumask)
>  	 */
>  }
>  
> +/**
> + * scx_bpf_get_idle_cpumask_node - Get a referenced kptr to the idle-tracking
> + * per-CPU cpumask of a target NUMA node.
> + *
> + * Returns an empty cpumask if idle tracking is not enabled, if @node is not
> + * valid, or running on a UP kernel.
> + */
> +__bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
> +{
> +	if (!static_branch_likely(&scx_builtin_idle_enabled)) {
> +		scx_ops_error("built-in idle tracking is disabled");
> +		return cpu_none_mask;
> +	}
> +	if (!static_branch_likely(&scx_builtin_idle_per_node)) {
> +		scx_ops_error("per-node idle tracking is disabled");
> +		return cpu_none_mask;
> +	}

Nub question: is it possible that scx_builtin_idle_per_node is enable,
but scx_builtin_idle_enabled not? From my naive perspective, we can't
enable per-node idle masks without enabling general idle masks. Or I
mislead it?

> +
> +	return get_idle_cpumask_node(node) ? : cpu_none_mask;
> +}
>  /**
>   * scx_bpf_get_idle_cpumask - Get a referenced kptr to the idle-tracking
>   * per-CPU cpumask of the current NUMA node.
>   *
> - * Returns NULL if idle tracking is not enabled, or running on a UP kernel.
> + * Returns an emtpy cpumask if idle tracking is not enabled, or running on a UP
> + * kernel.
>   */
>  __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask(void)
>  {
> @@ -7515,12 +7551,35 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask(void)
>  	return get_curr_idle_cpumask();
>  }
>  
> +/**
> + * scx_bpf_get_idle_smtmask_node - Get a referenced kptr to the idle-tracking,
> + * per-physical-core cpumask of a target NUMA node. Can be used to determine
> + * if an entire physical core is free.
> + *
> + * Returns an empty cpumask if idle tracking is not enabled, if @node is not
> + * valid, or running on a UP kernel.
> + */
> +__bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
> +{
> +	if (!static_branch_likely(&scx_builtin_idle_enabled)) {
> +		scx_ops_error("built-in idle tracking is disabled");
> +		return cpu_none_mask;
> +	}
> +	if (!static_branch_likely(&scx_builtin_idle_per_node)) {
> +		scx_ops_error("per-node idle tracking is disabled");
> +		return cpu_none_mask;
> +	}

Can you add vertical spacing between blocks?

Also, because you use this construction more than once, I think it
makes sense to make it a helper.

> +
> +	return get_idle_smtmask_node(node) ? : cpu_none_mask;
> +}
> +
>  /**
>   * scx_bpf_get_idle_smtmask - Get a referenced kptr to the idle-tracking,
>   * per-physical-core cpumask of the current NUMA node. Can be used to determine
>   * if an entire physical core is free.
>   *
> - * Returns NULL if idle tracking is not enabled, or running on a UP kernel.
> + * Returns an empty cumask if idle tracking is not enabled, or running on a UP
> + * kernel.
>   */
>  __bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask(void)
>  {
> @@ -7569,6 +7628,35 @@ __bpf_kfunc bool scx_bpf_test_and_clear_cpu_idle(s32 cpu)
>  		return false;
>  }
>  
> +/**
> + * scx_bpf_pick_idle_cpu_node - Pick and claim an idle cpu from a NUMA node
> + * @node: target NUMA node
> + * @cpus_allowed: Allowed cpumask
> + * @flags: %SCX_PICK_IDLE_CPU_* flags
> + *
> + * Pick and claim an idle cpu in @cpus_allowed from the NUMA node @node.
> + * Returns the picked idle cpu number on success. -%EBUSY if no matching cpu
> + * was found.
> + *
> + * Unavailable if ops.update_idle() is implemented and
> + * %SCX_OPS_KEEP_BUILTIN_IDLE is not set or if %SCX_OPS_KEEP_BUILTIN_IDLE is
> + * not set.
> + */
> +__bpf_kfunc s32 scx_bpf_pick_idle_cpu_node(int node, const struct cpumask *cpus_allowed,
> +				      u64 flags)
> +{

Sanity checks here?

> +	if (!static_branch_likely(&scx_builtin_idle_enabled)) {
> +		scx_ops_error("built-in idle tracking is disabled");
> +		return -EBUSY;
> +	}
> +	if (!static_branch_likely(&scx_builtin_idle_per_node)) {
> +		scx_ops_error("per-node idle tracking is disabled");
> +		return -EBUSY;
> +	}
> +
> +	return scx_pick_idle_cpu_from_node(node, cpus_allowed, flags);
> +}
> +
>  /**
>   * scx_bpf_pick_idle_cpu - Pick and claim an idle cpu
>   * @cpus_allowed: Allowed cpumask
> @@ -7705,14 +7793,18 @@ BTF_ID_FLAGS(func, scx_bpf_cpuperf_cap)
>  BTF_ID_FLAGS(func, scx_bpf_cpuperf_cur)
>  BTF_ID_FLAGS(func, scx_bpf_cpuperf_set)
>  BTF_ID_FLAGS(func, scx_bpf_nr_cpu_ids)
> +BTF_ID_FLAGS(func, scx_bpf_cpu_to_node)
>  BTF_ID_FLAGS(func, scx_bpf_get_possible_cpumask, KF_ACQUIRE)
>  BTF_ID_FLAGS(func, scx_bpf_get_online_cpumask, KF_ACQUIRE)
>  BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
>  BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask, KF_ACQUIRE)
> +BTF_ID_FLAGS(func, scx_bpf_get_idle_cpumask_node, KF_ACQUIRE)
>  BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask, KF_ACQUIRE)
> +BTF_ID_FLAGS(func, scx_bpf_get_idle_smtmask_node, KF_ACQUIRE)
>  BTF_ID_FLAGS(func, scx_bpf_put_idle_cpumask, KF_RELEASE)
>  BTF_ID_FLAGS(func, scx_bpf_test_and_clear_cpu_idle)
>  BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu, KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_pick_idle_cpu_node, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_pick_any_cpu, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
>  BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
> diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
> index 625f5b046776..9bbf6d5083b5 100644
> --- a/tools/sched_ext/include/scx/common.bpf.h
> +++ b/tools/sched_ext/include/scx/common.bpf.h
> @@ -59,14 +59,18 @@ u32 scx_bpf_cpuperf_cap(s32 cpu) __ksym __weak;
>  u32 scx_bpf_cpuperf_cur(s32 cpu) __ksym __weak;
>  void scx_bpf_cpuperf_set(s32 cpu, u32 perf) __ksym __weak;
>  u32 scx_bpf_nr_cpu_ids(void) __ksym __weak;
> +int scx_bpf_cpu_to_node(s32 cpu) __ksym __weak;
>  const struct cpumask *scx_bpf_get_possible_cpumask(void) __ksym __weak;
>  const struct cpumask *scx_bpf_get_online_cpumask(void) __ksym __weak;
>  void scx_bpf_put_cpumask(const struct cpumask *cpumask) __ksym __weak;
>  const struct cpumask *scx_bpf_get_idle_cpumask(void) __ksym;
> +const struct cpumask *scx_bpf_get_idle_cpumask_node(int node) __ksym __weak;
>  const struct cpumask *scx_bpf_get_idle_smtmask(void) __ksym;
> +const struct cpumask *scx_bpf_get_idle_smtmask_node(int node) __ksym __weak;
>  void scx_bpf_put_idle_cpumask(const struct cpumask *cpumask) __ksym;
>  bool scx_bpf_test_and_clear_cpu_idle(s32 cpu) __ksym;
>  s32 scx_bpf_pick_idle_cpu(const cpumask_t *cpus_allowed, u64 flags) __ksym;
> +s32 scx_bpf_pick_idle_cpu_node(int node, const cpumask_t *cpus_allowed, u64 flags) __ksym __weak;
>  s32 scx_bpf_pick_any_cpu(const cpumask_t *cpus_allowed, u64 flags) __ksym;
>  bool scx_bpf_task_running(const struct task_struct *p) __ksym;
>  s32 scx_bpf_task_cpu(const struct task_struct *p) __ksym;
> diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h
> index d56520100a26..587650490743 100644
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -125,6 +125,25 @@ bool scx_bpf_dispatch_vtime_from_dsq___compat(struct bpf_iter_scx_dsq *it__iter,
>  	false;									\
>  })
>  
> +#define __COMPAT_scx_bpf_cpu_to_node(cpu)                                       \
> +	(bpf_ksym_exists(scx_bpf_cpu_to_node) ?                                 \
> +	 scx_bpf_cpu_to_node(cpu) : 0)
> +
> +#define __COMPAT_scx_bpf_get_idle_cpumask_node(node)                            \
> +	(bpf_ksym_exists(scx_bpf_get_idle_cpumask_node) ?                       \
> +	 scx_bpf_get_idle_cpumask_node(node) :                                  \
> +	 scx_bpf_get_idle_cpumask())                                            \
> +
> +#define __COMPAT_scx_bpf_get_idle_smtmask_node(node)                            \
> +	(bpf_ksym_exists(scx_bpf_get_idle_smtmask_node) ?                       \
> +	 scx_bpf_get_idle_smtmask_node(node) :                                  \
> +	 scx_bpf_get_idle_smtmask())                                            \
> +
> +#define __COMPAT_scx_bpf_pick_idle_cpu_node(node, cpus_allowed, flags)          \
> +	(bpf_ksym_exists(scx_bpf_pick_idle_cpu_node) ?                          \
> +	 scx_bpf_pick_idle_cpu_node(node, cpus_allowed, flags) :                \
> +	 scx_bpf_pick_idle_cpu(cpus_allowed, flags))
> +
>  /*
>   * Define sched_ext_ops. This may be expanded to define multiple variants for
>   * backward compatibility. See compat.h::SCX_OPS_LOAD/ATTACH().
> -- 
> 2.47.1