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

[Andrea Righi <arighi@nvidia.com>]

On Wed, Dec 11, 2024 at 12:47:22PM -0800, Yury Norov wrote:
> On Wed, Dec 11, 2024 at 09:20:56PM +0100, Andrea Righi wrote:
> > On Wed, Dec 11, 2024 at 09:43:26AM -0800, Yury Norov wrote:
> > > 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. 
> > 
> > Ok, but what would you return with NUMA_NO_NODE, in theory we should return
> > a global system-wide cpumask, that doesn't exist with the per-node
> > cpumasks. Maybe just return cpu_none_mask? That's what I've done in the
> > next version, that seems safer than returning NULL.
>  
> To begin with, you can just disallow NUMA_NO_NODE for this interface.
> Put a corresponding comment or warning, and you're done.

Ok.

> 
> On the other hand, you can treat it as 'I don't care' hint and return
> a cpumask for any node that has idle CPUs.
> 
> Returning cpu_none_mask?.. OK, it's possible, but what does that
> bring? User will have to traverse empty mask just to find nothing.
> I'd rather disallow NUMA_NO_NODE than returning something useless.

I like the idea of returning a "random" node, or maybe
idle_masks[numa_node_id()]?

> 
> > > >  	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.
> > 
> > Maybe kernel/bpf/helpers.c? And name it bpf_cpu_to_node()?
> 
> Sure, even better
>  
> > > > +
> > > >  /**
> > > >   * 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?
> > 
> > In theory a BPF scheduler could set SCX_OPS_BUILTIN_IDLE_PER_NODE (without
> > SCX_OPS_KEEP_BUILTIN_IDLE) in .flags while implementing ops.update_idle().
> > 
> > In this way we would have scx_builtin_idle_enabled==false and
> > scx_builtin_idle_per_node==true, which doesn't make much sense, so we
> > should probably handle this case in validate_ops() and trigger an error.
> > 
> > Good catch!
> > 
> > > 
> > > > +
> > > > +	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?
> > 
> > You mean a blank between the two blocks, right?
> 
> Yes
> 
> > Anyway, ...
> > 
> > > 
> > > Also, because you use this construction more than once, I think it
> > > makes sense to make it a helper.
> > 
> > With a proper error check in validate_ops() we can just get rid of the
> > scx_builtin_idle_enabled block and simply check scx_builtin_idle_per_node.
> 
> But still, having a helper is better than opencoding the same 4-lines
> pattern again and again

Yep, makes sense. Will do that.

-Andrea